Omeprazole

Mechanism

Atazanavir dissolves and is absorbed less well when stomach acidity falls, and omeprazole suppresses acid secretion. Omeprazole 40 mg daily cut exposure to ritonavir-boosted atazanavir by 75%, and raising atazanavir to 400 mg did not make up for it. Without ritonavir, exposure fell by 94%. As a result, HIV treatment may lose virologic response and the virus may develop resistance.

Symptoms

No outward signs; the problem shows up in lab tests: HIV viral load rises on treatment, and the virus may become resistant to atazanavir.

Management

The combination is not recommended (omeprazole label; atazanavir label, European Medicines Agency (EMA)). If a proton pump inhibitor cannot be avoided, raise atazanavir to 400 mg with ritonavir 100 mg, keep omeprazole at no more than 20 mg a day and closely monitor the response to treatment, mainly viral load (omeprazole label; atazanavir label, EMA). Taking the drugs 12 hours apart does not prevent the drop in atazanavir levels (atazanavir label, EMA). For atazanavir without ritonavir, the label also advises against proton pump inhibitors (atazanavir label, EMA). The US Food and Drug Administration (FDA) does not recommend the combination in treatment-experienced patients; in treatment-naive patients it allows no more than omeprazole 20 mg taken about 12 hours before atazanavir 300 mg with ritonavir 100 mg (atazanavir FDA label). If acid suppression is needed, the atazanavir label allows H2-receptor antagonists with a dose limit: with atazanavir 300 mg plus ritonavir 100 mg and no tenofovir, no more than the equivalent of famotidine 20 mg twice daily (atazanavir label, EMA).

Mechanism

Omeprazole inhibits CYP2C19, which handles about 38% of citalopram metabolism. In therapeutic drug monitoring data, citalopram serum levels were about a third higher with omeprazole (+35%). The FDA caps citalopram at 20 mg/day with CYP2C19 inhibitors because QT prolongation is dose-dependent.

Symptoms

QT prolongation on the ECG. Clinically: dizziness, palpitations, syncope. Severe cases progress to torsade de pointes. The risk is higher with bradycardia, hypokalaemia, hypomagnesaemia, congenital long QT syndrome, recent myocardial infarction and decompensated heart failure.

Management

The pair is not banned, but the citalopram dose is capped: with omeprazole or another CYP2C19 inhibitor, no more than 20 mg/day (US Food and Drug Administration (FDA) citalopram label). The Spanish citalopram SmPC asks for caution with omeprazole, and the omeprazole SmPC warns that it can raise levels of drugs cleared through CYP2C19. Potassium and magnesium are corrected before citalopram is started, and in patients at risk of electrolyte disturbances they are checked at baseline and periodically (citalopram labels). If signs of arrhythmia appear, citalopram is stopped and an electrocardiogram (ECG) is done (citalopram SmPC).

Mechanism

Clopidogrel is a prodrug. It must be converted in the liver via CYP2C19 into its active antiplatelet metabolite. Omeprazole strongly inhibits CYP2C19: active metabolite levels drop by 45% and platelet inhibition weakens. The FDA issued a dedicated warning about this pair in 2009.

Symptoms

The patient feels no symptoms – the weakened antiplatelet effect is silent. The combination raises the risk of recurrent myocardial infarction, stent thrombosis, and stroke in patients with prior stenting or acute coronary syndrome.

Management

Replace omeprazole with pantoprazole, which has minimal effect on CYP2C19. Among other proton pump inhibitors, rabeprazole is an alternative. If a PPI is not required, switch to famotidine (an H2-blocker that does not affect CYP2C19) or stop acid suppression altogether.

Mechanism

Levothyroxine needs gastric acidity to dissolve before it can be absorbed. Omeprazole raises stomach pH and the dissolved fraction drops; the AEMPS label names proton pump inhibitors alongside calcium and iron. Unlike chelation, spacing the doses does not solve this one: pH stays high all day.

Symptoms

TSH rises after omeprazole is started and does not return to baseline even when the hormone timing changes. In atrophic gastritis or Helicobacter the drop is larger.

Management

If the inhibitor is staying, check TSH 6–8 weeks after starting it and adjust the levothyroxine dose on the result. When omeprazole stops, repeat the check: absorption recovers and the adjusted dose may now be too high. Also review whether the inhibitor is still needed, since many courses run on without a current indication.

Mechanism

Here the supplement does not obstruct the drug: the drug drains the patient of magnesium. The label records hypomagnesaemia as an adverse reaction of unknown frequency and warns that the severe form can come with hypocalcaemia and hypokalaemia. It appears after months or years of treatment, by which point nobody links the symptom to the stomach protector.

Symptoms

Cramps and muscle spasms, tremor, palpitations, dizziness, irritability. In severe cases tetany, arrhythmias and seizures. Low serum magnesium usually travels with low potassium and calcium that will not correct until magnesium is replaced.

Management

On long-term proton pump inhibitor therapy, measure serum magnesium before starting and periodically thereafter, especially alongside digoxin or a diuretic. Oral supplementation corrects most mild cases, but severe hypomagnesaemia often resolves only when the inhibitor is withdrawn. Also review whether it is still indicated: many of these courses run on out of inertia.

Mechanism

Omeprazole blocks the renal BCRP transporter and reduces methotrexate clearance. At oncologic methotrexate doses, plasma levels rise 2- to 3-fold, with reported severe nephrotoxicity and pancytopenia.

Symptoms

Mouth ulcers (mucositis), nausea, diarrhoea, hair loss. Blood counts fall: leukocytes, platelets, erythrocytes. Severe cases include fatal pancytopenia, nephrotoxicity, and hepatotoxicity.

Management

With high-dose (oncology) methotrexate, the Spanish label requires avoiding proton pump inhibitors, especially in renal impairment. Withhold omeprazole for the course and use an H2-receptor antagonist such as famotidine for gastric protection. Ranitidine is not an option: it has been suspended in the EU since 2020. With low-dose methotrexate (rheumatology, psoriasis) the combination is possible, but the labels warn about methotrexate accumulation, so monitor blood counts, creatinine and signs of toxicity such as mouth ulcers and diarrhoea, with extra care in reduced kidney function.

Mechanism

Omeprazole weakly blocks CYP3A4; alprazolam plasma levels may rise slightly. The effect is clinically weak.

Symptoms

Minimal changes: mild additional sedation in some patients. Older patients: rare ataxia.

Management

Dose adjustment is usually unnecessary. In older patients and prolonged therapy, monitor cognition and fall risk.

Mechanism

Omeprazole weakly blocks CYP3A4 – atorvastatin exposure rises modestly (by 10–20%). Clinically significant myopathy beyond baseline is rare.

Symptoms

Usually no changes. In patients with risk factors (older age, chronic kidney disease, hypothyroidism): muscle pain and weakness possible.

Management

The combination is acceptable without dose change. In patients with myopathy risk factors, check creatine kinase on muscle symptoms. Alternative PPI without CYP3A4 effect: pantoprazole or rabeprazole.

Mechanism

Ciprofloxacin absorbs better in acidic gastric environment. Omeprazole raises gastric pH — ciprofloxacin bioavailability drops by 20–30%. Clinically significant in infections with borderline pathogen sensitivity.

Symptoms

In severe infection: inadequate clinical response — persistent fever, symptoms, and inflammation markers 48–72 hours after antibiotic start.

Management

On the combination, take ciprofloxacin 2 hours before or 6 hours after omeprazole. If dosing cannot be separated, alternative antibiotics: amoxicillin-clavulanate for respiratory infections, a cephalosporin for UTIs. In urinary infections, ciprofloxacin still reaches adequate urine concentrations regardless of bioavailability.

Mechanism

Non-heme iron needs an acid environment to convert from ferric to ferrous, the form the gut absorbs. NIH ODS states that proton pump inhibitors can potentially reduce iron absorption. The practical consequence: iron deficiency anaemia in someone taking omeprazole responds to oral treatment worse than expected.

Symptoms

Ferritin that rises little or not at all after 8–12 weeks of well-taken oral iron, with no bleeding to explain it.

Management

Take iron with vitamin C, which assists the conversion gastric acid no longer performs. Keep at least 2 hours between iron and the inhibitor. If ferritin is still flat at 12 weeks, reconsider the route: with documented malabsorption, intravenous iron achieves what oral cannot.

Mechanism

Omeprazole inhibits CYP2C19, the enzyme responsible for sertraline metabolism. Sertraline levels rise by about 40%. Adverse effects may intensify: nausea, insomnia, sexual dysfunction.

Symptoms

Nausea, diarrhoea, insomnia or somnolence, tremor, anxiety, decreased libido. Symptoms appear in the first 1–2 weeks after starting omeprazole in patients on stable sertraline.

Management

For short omeprazole courses (2–4 weeks), no specific adjustment needed. For long-term therapy with significant adverse effects, reduce sertraline by 25–50% or switch to pantoprazole (minimal CYP2C19 effect).

Mechanism

Tacrolimus is metabolised by CYP3A4 in the liver and gut wall (tacrolimus SmPC). The tacrolimus SmPC classes omeprazole as a moderate or weak CYP3A4 inhibitor that can raise tacrolimus trough blood concentrations, sometimes rapidly. The omeprazole SmPC also reports higher tacrolimus concentrations with omeprazole but lists the mechanism as unknown.

Symptoms

Signs of tacrolimus excess: tremor, tingling in the limbs, headache, changes in mental status, rising creatinine and potassium. QT prolongation on the electrocardiogram is possible, with a risk of dangerous arrhythmia.

Management

The combination is acceptable with monitoring. From the first days of co-administration, check tacrolimus trough blood concentrations frequently and reduce the dose if needed; closely monitor renal function, the QT interval on the electrocardiogram (ECG) and other adverse effects (tacrolimus SmPC). The omeprazole SmPC also advises closer monitoring of tacrolimus concentrations and renal function (creatinine clearance), with tacrolimus dose adjustment if needed (omeprazole SmPC). The US Food and Drug Administration (FDA) gives the same approach: monitor trough concentrations and reduce the dose if needed (US tacrolimus label).

Mechanism

Food-bound vitamin B12 comes attached to protein and needs acid and pepsin to be released. Without acidity that release fails, and NIH ODS names gastric acid inhibitors among medications that affect B12 status. Supplemental B12 arrives already free and barely depends on pH, which is why the answer is to supplement rather than stop the drug.

Symptoms

Fatigue, tingling in hands and feet, difficulty concentrating, a smooth, sore tongue. Macrocytic anaemia comes late: neurological symptoms can precede an abnormal blood count.

Management

After 2–3 years on an inhibitor, measure serum B12 and, if borderline, methylmalonic acid. High-dose oral B12 works even without gastric acid, so the intramuscular route is rarely needed for this reason. Vegetarians and people over 65 start with less reserve and deserve earlier testing.

Mechanism

Proton pump inhibitors suppress gastric acidity and reduce B12 release from food (which requires HCl and pepsin). For short courses (up to 8 weeks), clinically insignificant. With long-term therapy (over 2 years), B12 deficiency risk is reported.

Symptoms

On long-term therapy: fatigue, pallor, paraesthesias in hands and feet, gait instability, memory decline.

Management

For short omeprazole courses, no adjustment needed. On long-term therapy (over 2 years), check B12 yearly. If it falls below 200 pg/mL, give cyanocobalamin 500–1000 mcg/day orally — passive diffusion absorption at high doses works regardless of acidity.

Mechanism

Omeprazole blocks the parietal cell proton pump and lowers the acidity against which ibuprofen leaves the mucosa unprotected by inhibiting prostaglandins. The ibuprofen label recommends exactly this: protective agents, misoprostol or proton pump inhibitors, in patients at risk. They neither compete for metabolism nor cancel each other out.

Symptoms

The pair is meant to prevent symptoms, not cause them. Worth knowing what it does not cover: omeprazole protects neither the kidney nor blood pressure nor the cardiovascular risk of the NSAID, and it does not prevent bleeding from the lower gut. Long PPI courses bring hypomagnesaemia and falling vitamin B12.

Management

The PPI is added with a history of ulcer or bleeding, older age, corticosteroids, anticoagulants or low-dose aspirin alongside. Protection does not extend the NSAID course: ibuprofen stays at the lowest effective dose for the shortest time, and omeprazole is stopped when the NSAID ends unless another indication exists.

Mechanism

Omeprazole weakly inhibits intestinal OCT1, the transporter mediating metformin absorption. Metformin bioavailability may rise slightly. Clinical significance is minimal.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

No dose adjustment needed. On long-term PPI therapy (over 2 years), check B12 yearly — both drugs may reduce its absorption.

Mechanism

Ticagrelor acts directly on the P2Y12 receptor and needs no CYP2C19 activation, so the classic clopidogrel-omeprazole problem does not carry over to this pair. The ticagrelor label focuses its warnings on bleeding risk and on aspirin maintenance doses, not on proton pump inhibitors.

Symptoms

No symptoms attributable to the pairing. Ticagrelor's own continue: bruising, gum or nose bleeding, dyspnoea appearing in the first weeks and usually mild. Black stools or coffee-ground vomit remain alarm signs.

Management

Omeprazole can stay alongside ticagrelor when there is a gastric indication. What does not change is bleeding surveillance and the rule against stopping the antiplatelet on your own, since early withdrawal raises cardiovascular risk.