Key takeaways:
Cimetidine (Tagamet HB) is an over-the-counter medication that treats and prevents heartburn and indigestion. A higher-dose version is available with a prescription for health conditions such as gastric and duodenal ulcers.
Cimetidine can interact with many medications, including warfarin (Jantoven), ketoconazole, and phenytoin (Dilantin). It can also interact with metronidazole (Flagyl), theophylline, and some beta blockers.
Before starting cimetidine, review your current medication list with a healthcare professional. They can help you identify any potential interactions and come up with a plan for managing them.
Save on related medications
Cimetidine (Tagamet HB) is a histamine-2 (H2) blocker that reduces stomach acid. It’s available over the counter (OTC) for heartburn or with a prescription for health conditions such as gastric and duodenal ulcers. But cimetidine is notorious for interacting with a lot of medications.
Cimetidine can affect other medications in a few different ways. Some interactions can increase the risk of side effects from other medications. Others can make certain medications less effective.
Sharing your current medication list with a healthcare professional will allow them to screen for cimetidine interactions before they become an issue. But it also helps to know about common culprits that you may encounter.
Cimetidine interactions at a glance
Some medications shouldn’t be taken with cimetidine, but many potential interactions can be managed with dosage adjustments or close monitoring. Even if you’re taking cimetidine for short-term or occasional relief, let your healthcare team know when you start or stop taking it.
While this is not an exhaustive list, examples of medications that can interact with cimetidine include:
Warfarin (Coumadin, Jantoven)
Phenytoin (Dilantin)
Lidocaine
Theophylline (Theo-24)
Ketoconazole
Beta blockers such as propranolol (Inderal LA) and metoprolol (Lopressor, Toprol XL)
Calcium channel blockers such as nifedipine (Procardia) and diltiazem (Cardizem)
Benzodiazepines such as diazepam (Valium) and chlordiazepoxide (Librium)
Tricyclic antidepressants, including imipramine and desipramine (Norpramin)
Metronidazole (Flagyl)
Below, we take a closer look at 10 cimetidine interactions and how to manage them.
1. Warfarin
Cimetidine blocks proteins in the liver that normally break down warfarin, a blood thinner. When taken together, cimetidine can cause warfarin to build up in your body, increasing the risk of bleeding.
If you need to take warfarin and cimetidine together, your prescriber will likely monitor your blood tests more closely. If needed, they’ll adjust your warfarin dose to ensure it’s safe and effective for you. But you should notify them right away if you have signs of bleeding, such as nosebleeds or bruising more easily.
2. Phenytoin
Phenytoin is an antiepileptic medication that’s used to treat and prevent seizures. At higher doses, cimetidine can raise phenytoin levels in the body and the risk of side effects. It’s not clear if this interaction is an issue with OTC cimetidine, which is taken at a lower dose.
Check with your healthcare team before taking cimetidine with phenytoin. They may say that it’s OK to take OTC cimetidine, or they may recommend alternatives. If you need to take cimetidine and phenytoin together, they may want to monitor your phenytoin levels more closely.
3. Lidocaine
Lidocaine is a local anesthetic that has many uses — from numbing the skin to treating abnormal heart rhythms. Cimetidine can raise lidocaine levels in the body and increase the risk of toxicity, especially if lidocaine is given as an injection or infusion. Because of this, you should let your care team know you’re taking cimetidine before having any type of medical procedure.
Cimetidine side effects: Do you experience diarrhea after taking cimetidine (Tagamet HB)? Learn about this and other potential cimetidine side effects and tips for managing them.
Is cimetidine available over the counter (OTC)? Get answers to this and other frequently asked questions about cimetidine.
Navigating options: There are a variety of OTC options for managing heartburn. Check out the top choices here.
Topical lidocaine products, such as patches, sprays, and creams, are less likely to interact with cimetidine. This is because a smaller amount of the medication is absorbed into the bloodstream. Though, it’s still a good idea to check with your healthcare team before using one of these products, especially on large areas of skin. Follow their directions to manage potential risks.
4. Theophylline
Theophylline is a medication that’s used to treat respiratory conditions such as asthma. It has a narrow therapeutic index, meaning that small changes in blood levels can have a big impact.
When taken together, cimetidine can raise levels of theophylline in your body, which can be very dangerous and lead to overdose. An overdose of theophylline can cause vomiting, seizures, and abnormal heart rhythms.
If you’re already taking cimetidine, your prescriber may start you on a lower dose of theophylline. Just remember to tell your prescriber if you stop taking cimetidine, so they can raise your theophylline dose as needed. Otherwise, theophylline may not work as well for you.
5. Ketoconazole
Some medications, such as ketoconazole, rely on an acidic environment for absorption. Cimetidine reduces stomach acid, which can affect how well you absorb these medications. This can make them less effective.
It’s possible to take cimetidine and ketoconazole together. But it’s recommended to take ketoconazole at least 2 hours before taking cimetidine. Otherwise, it may not work as well to treat your infection.
6. Beta blockers such as metoprolol and propranolol
Beta blockers are used to treat high blood pressure and other heart conditions. Cimetidine can interfere with how certain beta blockers — such as propranolol and metoprolol — are processed in the liver. As a result, these medications may build up in the body, increasing the risk of hypotension (low blood pressure) and bradycardia (slowed heart rate).
If you’re taking one of these beta blockers, your prescriber may recommend a different acid reducer. But if you need to take cimetidine, your prescriber may monitor your blood pressure and heart rate more closely. Let them know if your readings are lower than usual, or if you experience symptoms such as dizziness, lightheadedness, or fatigue.
7. Calcium channel blockers such as diltiazem
Cimetidine can raise the levels of certain calcium channel blockers, including nifedipine and diltiazem, in your body. These medications are used to treat high blood pressure and certain heart conditions. When their levels increase, so does the risk of side effects such as low blood pressure and a slowed heart rate.
If you’re taking one of these calcium channel blockers, your healthcare team may recommend an acid reducer that doesn’t interact.
8. Some benzodiazepines, such as diazepam
Benzodiazepines are used to treat conditions such as insomnia, anxiety, and seizures. Some, including diazepam and chlordiazepoxide, can interact with cimetidine. Cimetidine may cause these medications to stay in your system longer. This can raise the risk of side effects such as drowsiness and confusion.
If you’re taking a benzodiazepine, talk to your prescriber before taking cimetidine. They’ll consider the risk of side effects and, if needed, recommend a cimetidine alternative that doesn’t interact with your medication.
9. Tricyclic antidepressants, such as imipramine
Cimetidine can raise levels of tricyclic antidepressants (TCAs), a class of medications used to treat mental health conditions. This is more likely with certain TCAs, but it’s still important to be aware of the potential for an interaction with any of these medications. An interaction between cimetidine and a TCA can increase the risk of side effects such as drowsiness, dizziness, and an irregular heartbeat.
If you take a TCA and are considering starting cimetidine, your healthcare team may monitor you more closely, adjust your dose, or recommend a different acid reducer.
10. Metronidazole
Metronidazole is an antibiotic used to treat certain bacterial and parasitic infections. Cimetidine can slow down how fast your body processes metronidazole, which can increase the risk of side effects.
Talk to your care team before taking cimetidine and metronidazole together. They can make sure the combination is safe for you.
The bottom line
Cimetidine (Tagamet HB) is an over-the-counter acid reducer that’s also available by prescription. It can interact with many medications, including warfarin (Coumadin, Jantoven), phenytoin (Dilantin), and ketoconazole.
Review your current list of medications with your care team before taking cimetidine. They can help you identify any potential interactions and offer guidance on how to manage them if needed.
Why trust our experts?
References
Alembic Pharmaceuticals. (2024). Metoprolol tartrate - metoprolol tablet [package insert]. DailyMed.
Aurobindo Pharma. (2024). Cimetidine tablet, film coated [package insert]. DailyMed.
Moraczewski, J., et al. (2023). Tricyclic antidepressants.
National Institute of Diabetes and Digestive and Kidney Diseases. (2018). Cimetidine. LiverTox.
Nostrum Laboratories. (2021). Theophylline (anhydrous) - theophylline tablet, extended release [package insert]. DailyMed.
Rafi, J. A., et al. (1999). Effect of over-the-counter cimetidine on phenytoin concentrations in patients with seizures. Annals of Pharmacotherapy.
Shannon, M. (1998). Life-threatening events after theophylline overdose: A 10-year prospective analysis. JAMA Internal Medicine.
Sorkin, E. M., et al. (1983). Review of cimetidine drug interactions. Annals of Pharmacotherapy.
