Omeprazole Side Effects: What to Expect & When to Call Your Doctor

Omeprazole — sold as Prilosec OTC and by prescription — is a proton pump inhibitor (PPI) that reduces stomach acid production. It is used to treat acid reflux (GERD), stomach ulcers, H. pylori infection, and to protect the stomach lining in patients taking NSAIDs long-term. Omeprazole is available over the counter, which contributes to a common perception that it is entirely risk-free. In the short term it largely is — but long-term use carries meaningful nutritional and infectious risks that patients need to understand.

Common Side Effects

Short-term omeprazole is one of the best-tolerated medications in common use. Most side effects are mild and infrequent.

Serious Side Effects: When to Seek Help

Clostridioides difficile (C. diff) infection is a serious intestinal infection whose risk is elevated in PPI users, likely because stomach acid normally kills many bacteria before they reach the colon. C. diff causes severe, persistent diarrhea and requires specific antibiotic treatment. The risk is higher in hospitalized patients, the elderly, and those who have recently taken antibiotics.

Long-Term Risks: What the Research Shows

The long-term risks of PPIs are among the most discussed topics in primary care medicine. Several associations have emerged from large observational studies, though causality is not always firmly established. Understanding these risks is important for anyone who has been on a PPI for months or years, particularly if the original indication was never clearly defined or periodically reassessed.

Magnesium deficiency (hypomagnesemia): Long-term PPI use impairs magnesium absorption from the gut. This can cause muscle cramps, tremors, fatigue, and in severe cases, abnormal heart rhythms. The FDA issued a safety communication on this risk in 2011, recommending magnesium testing before starting long-term PPI therapy and periodically thereafter. If supplementation is needed, some forms of magnesium may be better absorbed than others — ask your doctor or pharmacist.

Vitamin B12 deficiency: Stomach acid is needed to cleave B12 from food proteins. By markedly suppressing acid, PPIs can reduce dietary B12 absorption over time. This is most relevant in patients with diets low in B12 (vegetarians, vegans) and those on very long-term therapy. Symptoms of B12 deficiency are subtle — fatigue, tingling in the hands and feet, and memory changes.

Fracture risk: Multiple observational studies have found a modest increase in hip, wrist, and spine fractures in long-term PPI users. The proposed mechanism involves impaired calcium absorption and altered osteoclast function. The absolute risk increase appears small, but it is clinically relevant in older patients with other osteoporosis risk factors. This is one reason routine bone density monitoring and adequate calcium and vitamin D intake are important in patients on long-term PPI therapy.

Rebound Acid Hypersecretion When Stopping

One of the least-discussed but most practically important aspects of omeprazole is what happens when you stop it. During therapy, the stomach compensates for reduced acid by proliferating acid-producing parietal cells and increasing the sensitivity of acid secretion pathways. When PPI therapy ends, especially abruptly, this upregulated system is suddenly unmasked — leading to a temporary surge in acid production that can cause heartburn worse than the original symptoms. This rebound effect typically peaks in the first 1–2 weeks after stopping and resolves over 2–4 weeks.

This rebound effect is one reason many people feel they "need" their PPI — they try to stop, feel terrible, and conclude they must stay on it. A gradual taper (decreasing frequency of use rather than abruptly stopping) reduces the rebound. Discuss a de-escalation plan with your doctor if you have been on omeprazole longer than intended.

Drug Interactions That Worsen Side Effects

Clopidogrel (Plavix): Omeprazole inhibits CYP2C19, which is needed to convert clopidogrel to its active form. This interaction may reduce clopidogrel's blood-thinning effectiveness. If you are on clopidogrel after a heart attack or stent, ask your cardiologist about this interaction — pantoprazole is generally considered a safer PPI alternative.

Methotrexate: PPIs may reduce kidney clearance of methotrexate, raising the risk of methotrexate toxicity, particularly at higher amounts used in cancer treatment.

Atazanavir and other HIV antiretrovirals: Some HIV medications require stomach acid for absorption and should not be taken with PPIs.

Frequently Asked Questions

Is it safe to take omeprazole every day long-term?

Long-term daily omeprazole use is appropriate for some medical conditions when prescribed and monitored by a physician. However, many people take PPIs longer than necessary. Long-term use is associated with magnesium deficiency, B12 deficiency, increased fracture risk, and higher C. difficile susceptibility. If you have been on omeprazole for more than 8 weeks and it was not prescribed for a long-term condition, discuss with your doctor whether it can be tapered.

Why does heartburn come back worse when I stop omeprazole?

This is called rebound acid hypersecretion. During therapy, the stomach upregulates its acid-producing capacity. When omeprazole is stopped, this surge is uncovered, causing a temporary worsening of heartburn that lasts 2–4 weeks. Tapering gradually rather than stopping abruptly helps reduce this effect.

Can omeprazole cause a magnesium deficiency?

Yes. Long-term PPI use (generally more than one year) can reduce magnesium absorption, causing hypomagnesemia. Symptoms include muscle cramps, tremors, irregular heartbeat, and fatigue. The FDA issued a safety communication about this in 2011. Your doctor may check magnesium levels if you have been on omeprazole long-term.