Clarithromycin and antacids: Timing Questions for Your Pharmacist

Last reviewed: October 2026

Picture aisle seven of almost any American pharmacy. A woman stands with a white prescription bag in one hand, a fresh course of clarithromycin inside, and a roll of Tums in the other. Her stomach has been sour since lunch. She reads the back of the Tums, then the sticker on the pill bottle, then looks toward the pharmacy counter. Her question is a simple one: clarithromycin and antacids, do they get along, and should she space them out? It deserves a straight answer. The more useful part, though, is what comes next. Once a pharmacist hears “clarithromycin,” the antacid is rarely the thing that worries them most.

This guide sorts the whole question into an interaction map with three zones. The green zone holds products where the clarithromycin label raises no flag. The amber zone covers acid reducers that change drug levels in ways that are known, measured, and often deliberate. The red zone is where the real trouble lives: the medicines clarithromycin can push to dangerous levels, some of which people take for symptoms they would describe as “just my stomach.” After the map comes a table, a list of timing questions to bring to the counter, and a short script for what to tell the pharmacist.

Key takeaways at a glance

Green: The current US clarithromycin prescribing information we reviewed on DailyMed does not list calcium carbonate or aluminum/magnesium antacids as interacting drugs, and it gives no instruction to separate doses from them. Ask your pharmacist if your own label or regimen says otherwise.

Amber: Omeprazole and clarithromycin raise each other’s blood levels. The label reports the figures, and H. pylori treatment packs pair them on purpose.

Red: Clarithromycin is a strong CYP3A inhibitor. Colchicine, simvastatin, lovastatin, some blood pressure medicines, QT-prolonging drugs, and warfarin are the combinations pharmacists actually watch for. The FDA also warned in 2018 about longer-term risk in people with heart disease.

Nothing below is a reason to change or skip a dose on your own. The map is meant to make your talk with the pharmacist about clarithromycin and antacids faster and more focused.

Green Zone: Clarithromycin and Antacids Like Tums, Maalox, and Mylanta

Start with what the woman in aisle seven is holding. Plain antacids neutralize acid already sitting in the stomach. The common ones are calcium carbonate (Tums and many store brands) and combinations of aluminum hydroxide and magnesium hydroxide (Maalox, Mylanta, and their generics). They act fast, wear off within hours, and don’t change how much acid the stomach makes.

What the clarithromycin label says, and what it leaves out

We read the current generic clarithromycin tablet prescribing information on DailyMed. Its drug interaction tables and clinical pharmacology section don’t mention calcium carbonate, aluminum hydroxide, magnesium hydroxide, or “antacids” as a group. It gives no spacing rule for them. The MedlinePlus patient page for clarithromycin doesn’t mention antacids either. It does name omeprazole and St. John’s wort as products to tell your clinician about.

Silence on a label is not the same as a formal statement that a combination is safe. It means the manufacturer and the FDA didn’t identify a clinically meaningful interaction worth listing. Pharmacists treat that as reassuring, not as a guarantee. Some older labels mentioned ranitidine bismuth citrate, a product no longer sold in the US; that wording doesn’t describe Tums.

The label does say something useful about the stomach environment. Clarithromycin immediate-release tablets can be taken with or without food. Food slightly delays absorption and raises the peak level by about 24 percent, but it doesn’t change the total amount absorbed. The extended-release tablet is different: MedlinePlus says to take it with food, once daily, and to swallow it whole. If you’re not sure which one you have, our explainer on clarithromycin immediate-release versus extended-release tablets walks through how to tell them apart.

How this differs from other antibiotics

When it comes to antibiotics and antacids, each drug behaves differently, so advice from a past prescription may not carry over. Some classes, such as tetracyclines and fluoroquinolones, bind to calcium, magnesium, and aluminum, and their labels set strict spacing windows. Clarithromycin’s macrolide cousin azithromycin is a partial case. Its label notes that aluminum- and magnesium-containing antacids lower peak levels, which we cover in our guide to azithromycin and antacids. Amoxicillin sits at the other end, with very little antacid concern. That post on amoxicillin and antacids makes a useful contrast. For clarithromycin and antacids, the label puts it closer to the amoxicillin end, even though it’s a macrolide like azithromycin.

Green-zone fine print

Green doesn’t mean “think about nothing.” A few practical points still apply.

• Your other medicines may care about antacids even if clarithromycin doesn’t. Thyroid hormone, iron, some HIV drugs, and certain other antibiotics can all be affected by calcium or magnesium. Your pharmacist looks at the whole list, not just one pair.
• Reaching for antacids several times a day is information. Heartburn that needs constant treatment, or stomach pain that started with the antibiotic, is worth mentioning. Clarithromycin commonly causes stomach upset and an altered or metallic taste. Our post on clarithromycin’s metallic taste explains that side effect.
• Kidney disease changes the antacid picture. Aluminum and magnesium can build up when the kidneys don’t clear them well, and clarithromycin doses are adjusted in severe kidney impairment. That combination of factors is a good reason to ask before buying anything over the counter.

Amber Zone: Acid Reducers Where Levels Shift

The amber zone is for products that change drug levels in measurable ways. Sometimes that’s helpful, sometimes it needs watching, and almost always it should be on your pharmacist’s radar. These are the acid reducers: proton pump inhibitors (PPIs), H2 blockers, the newer potassium-competitive acid blocker vonoprazan, and bismuth.

Omeprazole and other PPIs: a two-way street

Omeprazole (Prilosec and generics) is the PPI with the most clarithromycin data in the US label. In a study of healthy adults, clarithromycin 500 mg every 8 hours was given together with omeprazole 40 mg daily. The prescribing information reports changes in both directions:

• Clarithromycin raised omeprazole levels. At steady state, omeprazole’s peak concentration rose about 30 percent, its 24-hour exposure (AUC) about 89 percent, and its half-life about 34 percent.
• Omeprazole raised clarithromycin levels. Clarithromycin’s peak was about 10 percent higher, its trough about 27 percent higher, and its exposure over the dosing interval about 15 percent higher.
• The active metabolite rose more. 14-hydroxy clarithromycin (14-OH clarithromycin), which also fights bacteria, had peak, trough, and exposure increases of roughly 45, 57, and 45 percent.
• More drug reached the stomach lining. Clarithromycin levels in gastric tissue and mucus also went up with omeprazole.
• Acid suppression was a bit stronger. Average 24-hour gastric pH was 5.2 with omeprazole alone and 5.7 with both drugs, which the label attributes to higher omeprazole exposure.

The label files these under “no dose adjustment” or “use with caution.” The interaction isn’t considered dangerous for most people, but it’s real.

Why H. pylori regimens pair them on purpose

Here the amber zone turns intentional. Helicobacter pylori, the stomach bacterium behind many ulcers, is treated with several drugs at once. Raising stomach pH helps the antibiotics work and survive, and more clarithromycin in the gastric mucus is exactly the goal. The clarithromycin label itself lists PPI-based regimens: clarithromycin with omeprazole and amoxicillin, clarithromycin with lansoprazole and amoxicillin, and an older dual regimen of clarithromycin with omeprazole.

Over the years these combinations have been sold as convenience packs. Prevpac (lansoprazole-based) and Omeclamox-Pak (omeprazole-based) are examples. A newer option, the Voquezna Triple Pak, swaps the PPI for vonoprazan, a potassium-competitive acid blocker. Its labeling describes vonoprazan 20 mg, amoxicillin 1,000 mg, and clarithromycin 500 mg taken twice daily, 12 hours apart, with or without food, for 14 days. The FDA Drug Trials Snapshot for Voquezna Triple Pak and Dual Pak summarizes the trials behind its approval. The Voquezna labeling also notes that clarithromycin increases vonoprazan exposure and that both drugs can raise levels of other CYP3A4-processed medicines, which leads straight into the red zone.

One more piece of context: treatment preferences have shifted. The 2024 American College of Gastroenterology guideline on H. pylori treatment favors bismuth quadruple therapy when antibiotic susceptibility is unknown and reserves clarithromycin-containing regimens for infections known to be susceptible, because clarithromycin resistance has become common. If you’ve been handed a clarithromycin pack for H. pylori, it’s fair to ask your prescriber how the regimen was chosen. That’s a question about your treatment plan, not a reason to stop.

When a PPI is a separate, everyday medicine

Many people take a PPI daily for reflux and then get clarithromycin for something unrelated, such as a respiratory infection. The same two-way interaction applies. For most people that’s acceptable, but the pharmacist should know about it, especially if you also take other drugs that are processed by the same pathways. Long-term PPI use can also lower blood magnesium, according to FDA safety labeling for PPIs. Low magnesium is one of the factors that raises the risk of clarithromycin’s heart-rhythm effects. If you’ve taken a PPI for years, that’s worth a sentence at the counter.

The label gives detailed numbers only for omeprazole. Other PPIs differ in how they’re metabolized, so ask how yours fits.

H2 blockers: famotidine and cimetidine

H2 blockers such as famotidine (Pepcid) reduce acid production for about half a day. The current clarithromycin prescribing information we reviewed doesn’t list famotidine or cimetidine in its interaction tables. Famotidine is generally regarded as having few enzyme-based interactions. Cimetidine (Tagamet) is the older member of the class and inhibits several liver enzymes on its own, so pharmacists pay more attention to it when someone is already taking an enzyme inhibitor like clarithromycin. We’ve placed H2 blockers in amber because they change stomach pH and because cimetidine can complicate drug lists, not because the label flags a specific problem.

Bismuth: the other H. pylori backbone

Bismuth subsalicylate (Pepto-Bismol and similar products) coats the stomach and has activity against H. pylori. It’s the anchor of bismuth quadruple therapy, which usually combines bismuth, a PPI, tetracycline, and metronidazole, with no clarithromycin at all. That’s why bismuth and clarithromycin rarely appear on the same prescription. The clarithromycin label doesn’t list bismuth subsalicylate as an interacting drug. Bismuth subsalicylate does contain salicylate, which matters with blood thinners, aspirin allergy, or use in children and teens, and it can harmlessly turn the tongue and stool black. If you’re tempted to add Pepto-Bismol to a clarithromycin course for an upset stomach, check with the pharmacist first.

Red Zone: The Interactions Pharmacists Actually Worry About

This is the zone the woman in aisle seven didn’t ask about, and the one her pharmacist will think about first. Clarithromycin strongly inhibits CYP3A4, a liver and gut enzyme that breaks down a large share of common medicines, and it also inhibits P-glycoprotein (P-gp), a transporter that pumps some drugs out of cells. When those pathways are blocked, other drugs can build up quickly, sometimes to toxic levels, within days.

This zone is easy to miss because several red-zone drugs hide behind everyday complaints. A gout flare is “my foot acting up.” A statin is “my cholesterol pill.” Nausea from a drug building up gets blamed on the antibiotic or the antacid.

Colchicine

Colchicine (Colcrys, Mitigare, Gloperba, and generics, plus the low-dose Lodoco used for heart protection) treats and prevents gout flares and is used for familial Mediterranean fever and some heart conditions. It depends on both CYP3A4 and P-gp, so clarithromycin hits it twice. The clarithromycin label makes the combination contraindicated in people with kidney or liver impairment. For people with normal kidney and liver function, it calls for a reduced colchicine dose and monitoring for toxicity. Serious and fatal colchicine toxicity has been reported with this pairing. The early signs of colchicine toxicity are diarrhea, nausea, vomiting, and stomach pain, which is exactly why it hides behind “stomach” complaints. Muscle weakness or pain and unusual bleeding or infections can follow. Don’t adjust colchicine yourself. Make sure the prescriber and pharmacist both know you take it.

Simvastatin and lovastatin

These two cholesterol medicines are broken down almost entirely by CYP3A4. With clarithromycin on board, their levels can rise enough to cause myopathy and rhabdomyolysis, a breakdown of muscle tissue that can damage the kidneys. The label makes clarithromycin contraindicated with lovastatin and simvastatin, including combination products that contain them. Prescribers usually handle this by pausing the statin for the length of the antibiotic course or choosing a different antibiotic. That decision belongs to them. Other statins depend less on CYP3A4, and the label has its own guidance for atorvastatin and pravastatin. Unexplained muscle pain or weakness, especially with dark urine, needs prompt attention.

Certain calcium channel blockers

Calcium channel blockers that are processed by CYP3A4, including amlodipine, diltiazem, nifedipine, felodipine, and verapamil, are among the most common blood pressure and heart-rate medicines in the US. The clarithromycin label warns of low blood pressure and acute kidney injury when these drugs are combined, with older adults at higher risk. For verapamil specifically, the label notes reports of hypotension, slow heart rhythms, and lactic acidosis. Report dizziness, fainting, or reduced urination.

QT-prolonging drugs and heart rhythm

Clarithromycin can lengthen the QT interval, part of the heart’s electrical cycle, and has been linked to infrequent cases of arrhythmia, including torsades de pointes. The risk adds up when other QT-prolonging drugs are involved. The label makes pimozide and cisapride contraindicated, calls for ECG monitoring with antiarrhythmics such as quinidine and disopyramide, and lists postmarketing reports of QT prolongation with quetiapine. Some antidepressants, antipsychotics, antinausea drugs such as ondansetron, and other antiarrhythmics can add to the effect as well. MedlinePlus asks patients to tell their doctor about a prolonged QT interval, irregular heartbeat, low potassium or magnesium, or coronary artery disease. A fast, pounding, or irregular heartbeat, or fainting, during treatment needs urgent care.

Warfarin and other blood thinners

The clarithromycin label warns of serious bleeding and significant rises in INR and prothrombin time when it’s used with warfarin, and recommends checking INR frequently. Some newer anticoagulants are also affected by CYP3A4 and P-gp inhibitors, and the label addresses those separately. Watch for unusual bruising, bleeding gums, blood in urine, or black, tarry stools (which bismuth can also cause).

Other contraindicated or high-alert combinations

The label also makes clarithromycin contraindicated with ergotamine and dihydroergotamine (migraine drugs) and lomitapide, and in people who had jaundice or liver problems with previous clarithromycin use. The Voquezna Triple Pak labeling adds lurasidone and rilpivirine-containing products to its contraindications. The interaction tables also cover sedatives such as midazolam and triazolam, some diabetes medicines (low blood sugar has been reported), and drugs for HIV, transplant, and erectile dysfunction. You don’t need to memorize this. Your pharmacist needs a complete list.

The FDA’s 2018 heart-disease warning

In February 2018 the FDA issued a Drug Safety Communication on clarithromycin in patients with heart disease. It was based on 10-year follow-up of the CLARICOR trial, in which people with stable coronary heart disease took a two-week course of clarithromycin or placebo. More deaths occurred in the clarithromycin group, and the difference became apparent one year or more after treatment ended. The prescribing information now reports 866 deaths (40 percent) in the clarithromycin group versus 815 (37 percent) with placebo, a hazard ratio of 1.10. The FDA couldn’t explain the finding, and observational studies were mixed. It still added a warning and advised prescribers to weigh the risks even for short courses and to consider other antibiotics for patients with heart disease.

For patients, the FDA’s advice was clear. Tell any clinician who prescribes an antibiotic that you have heart disease. Don’t stop your heart medicines or the antibiotic on your own. Get emergency care for chest pain, shortness of breath, or stroke symptoms such as slurred speech or weakness on one side.

The Three Zones in One Table

Zone	Examples	What the label or evidence says	What to do
Green	Calcium carbonate (Tums); aluminum/magnesium hydroxide (Maalox, Mylanta)	Not listed as interacting in the current US clarithromycin label we reviewed; no spacing instruction given	Mention them; ask whether your other medicines need spacing from antacids
Amber	Omeprazole and other PPIs; vonoprazan; famotidine, cimetidine; bismuth subsalicylate	Omeprazole and clarithromycin raise each other’s levels (omeprazole AUC up about 89%, 14-OH clarithromycin AUC up about 45%); paired on purpose in H. pylori packs	Tell the pharmacist about every acid reducer, including OTC and long-term use
Red	Colchicine; simvastatin, lovastatin; CYP3A4 calcium channel blockers; QT-prolonging drugs; warfarin; ergotamine; pimozide	Contraindications or serious warnings: muscle breakdown, colchicine toxicity, low blood pressure, kidney injury, arrhythmia, bleeding	Make sure the prescriber and pharmacist know before the first dose; never adjust these drugs yourself
Red (patient factor)	Coronary heart disease; prolonged QT; low potassium or magnesium; kidney or liver disease	FDA 2018 warning on long-term mortality in heart disease; QT and dosing warnings	Say it out loud at the counter, even if it’s in your chart

Timing Questions to Bring to the Counter

Timing questions are where pharmacists can help most, because the right answer depends on your exact products and schedule. Here are ten worth asking, roughly from simplest to most involved.

1. “Does my clarithromycin need any spacing from Tums or Maalox?” Based on the current label, the answer is usually no for clarithromycin itself. Asking still lets the pharmacist check the rest of your list.
2. “Is this the immediate-release or extended-release tablet, and does it go with food?” Immediate-release can be taken with or without food. Extended-release goes with food and is swallowed whole.
3. “How far apart should my doses be?” Immediate-release is usually every 12 hours (every 8 hours in some regimens), and extended-release once every 24 hours. Your label is what counts.
4. “I take a daily PPI. Do I keep taking it at the same time?” Many people do, but the two-way interaction makes this worth confirming.
5. “My H. pylori pack has three or four drugs. Which ones go together, and when?” Combination packs come with their own dosing cards. Ask the pharmacist to walk you through the first day.
6. “Can I use Pepcid or Pepto-Bismol for heartburn during this course?” Each one raises different questions, from cimetidine’s enzyme effects to bismuth’s salicylate content.
7. “Should my statin, colchicine, or blood pressure medicine be changed while I’m on this?” Only your prescriber can make that change. The pharmacist can flag it and contact them.
8. “I take warfarin. When should my INR be checked?” The label recommends frequent INR monitoring with this combination.
9. “How long after my last clarithromycin dose do the interactions last?” Enzyme inhibition doesn’t disappear the moment you take the last pill. Ask when it’s appropriate to restart anything that was paused.
10. “What should I do if I miss a dose?” MedlinePlus advises taking it as soon as you remember, skipping it if the next dose is almost due, and never doubling up. Our clarithromycin missed-dose guide covers this in more detail.

What to Tell the Pharmacist

Questions only help if the pharmacist has the full picture. Many dangerous combinations get caught simply because someone mentioned a medicine they didn’t think counted. Use this as a short script.

Say these out loud, even if they’re already in your chart:

1. Every acid product you use: antacids, Pepcid, Prilosec or another PPI, Pepto-Bismol, including “only now and then” use.

2. Any gout medicine (especially colchicine), cholesterol medicine, blood pressure or heart-rhythm medicine, or blood thinner.

3. Migraine medicines, sleep or anxiety medicines, antidepressants, antipsychotics, and antinausea drugs.

4. Heart disease of any kind, a history of a long QT interval or irregular heartbeat, and any recent lab result showing low potassium or magnesium.

5. Kidney or liver problems, and any past reaction to clarithromycin, erythromycin, or azithromycin.

6. Supplements and herbal products, especially St. John’s wort.

7. Whether you’re pregnant, could be pregnant, or are breastfeeding.

Bring a written list, the bottles, or phone photos of the labels. If you’d rather talk through your list before picking anything up, our contact page explains how to reach the Mediclox team.

Then the stewardship part, which applies no matter which zone your medicines fall in. Take clarithromycin exactly as prescribed and finish the course even if you feel better, unless your prescriber tells you to stop. MedlinePlus notes that stopping early can leave the infection undertreated and encourage resistance. Don’t save leftover tablets for another illness or share them. Clarithromycin resistance is one reason H. pylori guidelines have shifted away from it.

Back in aisle seven, the most likely outcome is undramatic. The pharmacist confirms that the Tums are fine for occasional heartburn, asks what else she takes, and learns that she has been on simvastatin for years. That second question is what the conversation was really for. Asking about clarithromycin and antacids opened the door.

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Frequently Asked Questions

Do I need to wait two hours between Tums and clarithromycin?

The current US clarithromycin prescribing information we reviewed doesn’t give a spacing interval for calcium carbonate or other plain antacids. Two-hour rules apply to other antibiotics, like tetracyclines and fluoroquinolones. Your pharmacist may still suggest spacing because of another medicine you take, so ask.

Why would my doctor prescribe omeprazole with clarithromycin if they interact?

For H. pylori infections, the interaction helps. Omeprazole lowers stomach acid and increases clarithromycin and its active metabolite in the blood and in the stomach lining. The clarithromycin label lists omeprazole-based regimens as approved H. pylori treatments.

Is Pepcid a better choice than Prilosec while I’m on clarithromycin?

It depends on why you need acid control and what else you take. The label has detailed interaction data for omeprazole but doesn’t list famotidine. If a PPI is part of a prescribed H. pylori regimen, don’t swap it. For occasional heartburn, ask your pharmacist which option fits your medication list.

Which stomach symptoms during clarithromycin should not be treated with an antacid?

Severe or watery diarrhea, especially with fever or blood, can signal C. difficile infection and needs a call to your prescriber. Nausea, vomiting, and diarrhea in someone taking colchicine can be early colchicine toxicity. Yellowing of the skin or eyes, dark urine, or upper-right belly pain can point to liver problems. None of these are antacid situations.

Does the FDA heart warning mean I shouldn’t take clarithromycin if I have heart disease?

Not automatically. The FDA asked prescribers to weigh benefits and risks and to consider other antibiotics for people with heart disease. Make sure your prescriber knows your history, and don’t stop the antibiotic or your heart medicines without talking to them first.

Does it matter whether my clarithromycin is the extended-release version?

For antacids, the label gives no special instruction for either form. For food, it does matter: immediate-release can be taken with or without meals, and extended-release should be taken with food and swallowed whole. The drug-interaction warnings apply to both.

Can I take my statin on the days I’m not taking clarithromycin?

Clarithromycin is usually taken every day of the course, and its enzyme effects carry over between doses. Lovastatin and simvastatin are contraindicated with it. Any pause or switch, and when to restart, should come from your prescriber.

References

• DailyMed: Clarithromycin tablets, film coated, prescribing information (National Library of Medicine)
• MedlinePlus: Clarithromycin (National Library of Medicine)
• FDA Drug Safety Communication: Potential increased long-term risks with clarithromycin in patients with heart disease (February 2018)
• FDA Drug Trials Snapshots: Voquezna Triple Pak and Voquezna Dual Pak
• ACG Clinical Guideline: Treatment of Helicobacter pylori Infection (American Journal of Gastroenterology, 2024)

This article is for general information and isn’t a substitute for advice from your prescriber or pharmacist. Don’t start, stop, or change the timing or dose of any medicine based on this article. If you have chest pain, fainting, a racing or irregular heartbeat, severe diarrhea, or signs of an allergic reaction, seek medical care right away.