Omeprazole interacts with a surprisingly long list of medications, and the interactions fall into two main categories: drugs that become less effective because omeprazole reduces stomach acid, and drugs that build up to dangerous levels because omeprazole interferes with how the liver breaks them down. Some of these interactions are serious enough that the FDA has issued specific warnings against combining them.
Blood Thinners: The Clopidogrel Warning
The most well-known interaction is between omeprazole and clopidogrel (Plavix), a blood thinner prescribed after heart attacks, stent placement, and strokes. Clopidogrel is a prodrug, meaning your body has to convert it into its active form before it works. That conversion depends on a liver enzyme called CYP2C19, and omeprazole blocks that enzyme. The result: clopidogrel can’t do its job of preventing blood clots.
The FDA label for Plavix explicitly states to avoid taking it with omeprazole or esomeprazole because both “significantly reduce the antiplatelet activity” of the drug. This holds true even if you space the doses 12 hours apart. If you need acid suppression while on clopidogrel, pantoprazole is generally considered a safer alternative because it has a weaker effect on CYP2C19.
HIV Medications
Several HIV drugs depend on an acidic stomach environment to dissolve and absorb properly. Omeprazole raises stomach pH dramatically, which can slash the amount of these medications that reaches your bloodstream. The consequences are not minor: subtherapeutic drug levels can lead to treatment failure and drug resistance.
Atazanavir is the clearest example. When taken with omeprazole 40 mg, atazanavir blood levels drop by 76%. Even at a lower omeprazole dose of 20 mg given 12 hours before atazanavir, levels still fall by 42%. Current HIV treatment guidelines restrict omeprazole to no more than 20 mg daily in patients who have never used a protease inhibitor before, and they prohibit the combination entirely in patients with prior protease inhibitor experience. Nelfinavir and rilpivirine are also affected, with guidelines recommending against combining them with any proton pump inhibitor.
Cancer and Immune-Suppressing Drugs
High-dose methotrexate, used in cancer treatment and sometimes for autoimmune conditions, is one of the more dangerous interactions. Omeprazole slows the kidneys’ ability to clear methotrexate from the body, causing it to accumulate to toxic levels. Patients in documented cases experienced kidney damage, dangerously low blood cell counts, severe mouth sores, and prolonged muscle pain lasting days. For patients receiving high-dose methotrexate, switching to a different type of acid reducer is the standard recommendation.
Tacrolimus, an immune suppressant taken by organ transplant recipients, also interacts with omeprazole through the same liver enzyme pathway. The combination can cause tacrolimus levels to rise unpredictably, increasing the risk of toxicity. Transplant patients starting or switching PPIs typically need close monitoring of their tacrolimus blood levels. Mycophenolate, another transplant drug, faces a different problem: omeprazole reduces its absorption by raising stomach pH, potentially leaving patients underprotected against organ rejection.
Certain Cancer-Targeted Therapies
A growing number of oral cancer drugs require stomach acid for proper absorption, and omeprazole can render them ineffective. The list includes dasatinib, erlotinib, gefitinib, nilotinib, bosutinib, neratinib, pazopanib, and acalabrutinib. For most of these, current guidelines recommend against combining them with any PPI. If you’re prescribed one of these medications, your oncologist will likely switch you to a different approach for managing acid reflux or heartburn.
Antifungal Medications
Ketoconazole, itraconazole, and posaconazole (in its oral suspension form) all need an acidic stomach to dissolve properly. Omeprazole can reduce their absorption enough to make them ineffective against serious fungal infections. For itraconazole, administering it with a non-diet cola (the acidity helps) and separating it from omeprazole by at least two hours is sometimes used as a workaround, but avoiding the combination when possible is preferred.
Voriconazole presents the opposite problem. Instead of being poorly absorbed, it is broken down by CYP2C19, the same enzyme omeprazole inhibits. This can cause voriconazole to accumulate in the body, increasing the risk of side effects like visual disturbances and liver problems.
Seizure and Anxiety Medications
Diazepam (Valium) is broken down by CYP2C19, so omeprazole slows its metabolism and can lead to excessive sedation and toxicity. The interaction is well-documented enough that switching to pantoprazole or another PPI with less enzyme inhibition is a standard recommendation.
Carbamazepine, used for epilepsy, also competes with omeprazole for the same metabolic pathway, and combining them can push carbamazepine levels into the toxic range. Other seizure medications affected include clobazam, brivaracetam, and lacosamide, all of which rely on CYP2C19 for metabolism. Clozapine, an antipsychotic, presents yet another wrinkle: omeprazole actually speeds up its breakdown through a different enzyme, potentially making it less effective.
Heart Medication: Digoxin
Omeprazole can increase the absorption of digoxin, a medication used for heart failure and certain irregular heart rhythms, by altering stomach pH. Since digoxin has a narrow therapeutic window (meaning the difference between an effective dose and a toxic one is small), even modest increases in blood levels can cause nausea, visual changes, and dangerous heart rhythm disturbances. Patients on both medications typically need periodic monitoring of digoxin levels.
Certain Antibiotics
Several oral antibiotics depend on stomach acid for absorption. Cefditoren, cefpodoxime, and cefuroxime, all members of the cephalosporin family, can have reduced effectiveness when taken with omeprazole. If you’re prescribed one of these for an infection, your doctor may choose a different antibiotic or a different acid-reducing strategy.
Iron Supplements and Nutrient Absorption
Omeprazole reduces the absorption of non-heme iron (the type found in supplements and plant foods) by raising stomach pH. Iron needs an acidic environment to convert into a form your body can absorb. If you’re taking iron supplements for anemia, you may need higher doses or, in some cases, intravenous iron to compensate.
Long-term omeprazole use also affects other nutrients. Vitamin B12 levels should be checked after more than two years of continuous use, or sooner if you develop symptoms like fatigue, numbness, or tingling. Magnesium levels deserve monitoring as well, especially if you’re also taking diuretics (water pills), since both medications independently lower magnesium. Periodic blood tests for magnesium, B12, and iron are recommended for anyone on omeprazole long-term.
St. John’s Wort and Herbal Products
St. John’s Wort, a popular herbal supplement for mood, works in the opposite direction of most interactions on this list. Instead of being affected by omeprazole, it dramatically reduces omeprazole’s effectiveness. After 14 days of St. John’s Wort use, peak omeprazole blood levels dropped by 38 to 50%, and overall drug exposure fell by a similar margin. This happens because St. John’s Wort revs up the liver enzymes that break omeprazole down. The practical consequence is that your acid reflux or ulcer treatment may simply stop working. Ginkgo biloba and a traditional Chinese herbal formula called Yin Zhi Huang have similar effects.
Antidepressants
Certain antidepressants are metabolized through CYP2C19 and can be affected by omeprazole’s enzyme-blocking activity. Citalopram (Celexa) is the most notable example. When omeprazole slows its breakdown, citalopram levels can rise, increasing the risk of side effects like nausea, dizziness, and in rare cases, heart rhythm changes. Tricyclic antidepressants such as amitriptyline and imipramine are also metabolized through this pathway and may accumulate when combined with omeprazole.
Hepatitis C Treatments
Some direct-acting antiviral combinations used for hepatitis C have specific instructions around PPI use. The combination of sofosbuvir and velpatasvir (Epclusa) can be taken with omeprazole only if the PPI dose doesn’t exceed 20 mg, the antiviral is taken with food, and it is given four hours before the omeprazole dose. The combination of ledipasvir and sofosbuvir (Harvoni) can be taken simultaneously with a PPI under fasting conditions, but exceeding recommended PPI doses may reduce antiviral effectiveness.