Omeprazole is one of the most widely used medications in the world, and most people tolerate it well in the short term. The common side effects are mild: headache, stomach pain, nausea, diarrhea, constipation, gas, and dizziness. These typically resolve on their own. But omeprazole was designed for short courses of treatment, and the side effects that concern doctors most are the ones that emerge after months or years of daily use.
Common Short-Term Side Effects
The everyday side effects of omeprazole are digestive in nature, which makes sense for a drug that works by shutting down acid production in the stomach. Headache is the most frequently reported complaint, followed by abdominal pain, diarrhea, nausea, and gas. Some people also experience constipation, dizziness, or drowsiness. These side effects are generally mild and don’t require stopping the medication.
Nutrient Deficiencies From Prolonged Use
Your stomach acid does more than break down food. It helps your body absorb certain vitamins and minerals, and when omeprazole suppresses that acid for a long time, nutritional gaps can develop. The three nutrients most affected are vitamin B12, magnesium, and iron.
Vitamin B12 deficiency is the best-studied concern. Taking omeprazole daily for a year or more increases the risk, because stomach acid is needed to release B12 from food so the body can absorb it. B12 deficiency can cause fatigue, numbness or tingling in the hands and feet, difficulty with balance, and memory problems. Magnesium levels can also drop, sometimes severely enough to cause muscle cramps, irregular heartbeat, or seizures. Iron absorption is similarly impaired, which can contribute to anemia over time.
If you’ve been on omeprazole for more than a year, it’s reasonable to have your B12 and magnesium levels checked periodically.
Bone Fracture Risk
Long-term omeprazole use is associated with a modest increase in hip, wrist, and spine fractures. Two large analyses of published studies put the increased risk at 10 to 40% above baseline, with the highest risk seen in people taking high doses for more than a year. The mechanism likely ties back to calcium absorption: without enough stomach acid, your body has a harder time taking in calcium from food, which can weaken bones over time. This is most relevant for older adults and anyone already at risk for osteoporosis.
Increased Risk of Certain Infections
Stomach acid serves as a barrier against bacteria you swallow. When omeprazole lowers that acid, some infections become more likely. The most concerning is C. difficile, a bacterial infection that causes severe diarrhea and can be dangerous in older or hospitalized patients. One study found that omeprazole use was associated with a roughly 3.6 times higher odds of recurrent C. difficile infection. There is also a recognized link between long-term PPI use and a higher rate of pneumonia, likely because reduced stomach acid allows bacteria to travel upward into the airways.
Kidney Effects
Omeprazole can, in rare cases, cause a type of kidney inflammation called acute interstitial nephritis. This can happen weeks to months after starting the drug and may show up as unexplained changes in urination, swelling, or fatigue. If caught early and the medication is stopped, it’s usually reversible.
Beyond this acute reaction, several large studies have found a small but consistent link between long-term PPI use and chronic kidney disease. Compared to people not taking PPIs, users showed a 10 to 39% higher risk of developing chronic kidney disease, depending on the study. These are observational findings, meaning they don’t prove omeprazole directly causes kidney damage, but the pattern has been consistent enough that doctors factor it into decisions about long-term use.
What About Dementia and Heart Disease?
You may have seen headlines linking omeprazole to dementia or cardiovascular problems. Earlier observational studies raised this concern, but stronger evidence has since pushed back on it. A large prospective study of nearly 19,000 adults aged 65 and older found no association between PPI use and dementia, cognitive impairment, or decline in cognitive test scores over time. Yale Medicine researchers have noted that the earlier findings were likely coincidental rather than showing a true cause-and-effect relationship. The same pattern holds for heart disease: initial signals from observational data have not held up under more rigorous analysis.
Interactions With Other Medications
Omeprazole interferes with how the body processes certain drugs. The most important interaction is with clopidogrel, a blood thinner commonly prescribed after heart attacks and stent placement. Omeprazole blocks an enzyme the body needs to activate clopidogrel, reducing the active form of the drug by about 40% and cutting its ability to prevent blood clots by roughly 30%. This is a clinically significant interaction. If you take clopidogrel, your doctor will typically switch you to a different acid-reducing medication.
Omeprazole can also affect levels of methotrexate (used for autoimmune conditions and cancer) and several antifungal medications that require stomach acid for absorption.
Rebound Symptoms When Stopping
If you’ve taken omeprazole daily for more than a few weeks, stopping abruptly can trigger a temporary surge of stomach acid that’s actually worse than what you had before starting. This rebound effect causes heartburn, regurgitation, and upper stomach discomfort. It happens because your stomach compensates for the acid suppression by ramping up its acid-producing machinery, and when the drug is removed, all that machinery fires at once.
The American Gastroenterological Association notes that either gradual tapering or abrupt discontinuation is acceptable, but patients should expect transient symptoms either way. Some people find it easier to step down gradually, cutting the dose in half for a week or two before stopping entirely. The rebound symptoms are temporary, not a sign that you still need the medication.