What happens when you take omeprazole with calcium?
The interaction depends entirely on which form of calcium you are taking. Calcium carbonate, the most common and cheapest supplemental form (Tums, Caltrate, OsCal, most multivitamins), is poorly soluble at neutral pH and needs gastric acid to dissolve and ionize before it can be absorbed in the small intestine. Omeprazole suppresses that acid by 90% or more, so calcium carbonate absorption drops sharply on an empty stomach.
Calcium citrate, by contrast, is already in an ionized, soluble form. Its absorption does not depend on stomach acid. Multiple studies show that in PPI users, calcium citrate absorption is preserved while calcium carbonate absorption (especially fasting) can be reduced by 40-60%. Taking calcium carbonate with food partially rescues absorption because meals stimulate residual acid output, but the difference between forms remains significant in PPI users.
Why is this important?
Calcium is the structural backbone of bone. Chronic underabsorption, combined with the secondary hyperparathyroidism it can trigger, accelerates bone turnover and reduces bone mineral density over years. The clinical consequence is fracture risk.
In 2010, the FDA updated PPI labeling to warn of a possible increased risk of fractures of the hip, wrist, and spine with high-dose, long-term (more than one year) PPI use. This was based on multiple observational studies and meta-analyses. A meta-analysis of 11 international studies found PPI use associated with a roughly 30% increased relative risk of hip fracture, 56% increased risk of spine fracture, and 16% increase in any-site fracture. The absolute risk increase is small for any individual, but at population scale it matters, especially in postmenopausal women and older adults.
The fracture risk is probably multifactorial: impaired calcium absorption is one mechanism, but PPIs also affect osteoclast function, magnesium status, and possibly vitamin B12-related neurologic function (which contributes to falls). Hip fracture in an older adult is a sentinel event with high mortality, so the cumulative risk matters.
What should you do?
- Switch to calcium citrate if you need a supplement. Citracal and other calcium citrate brands absorb well regardless of stomach acid. Target 1000-1200 mg elemental calcium per day from diet plus supplement combined, not from supplement alone.
- Take calcium with food. Even citrate absorbs better with a meal, and food stimulates whatever residual acid you have. Avoid taking calcium on an empty stomach.
- Split your dose. The gut absorbs no more than about 500 mg elemental calcium at a time efficiently. Two 500 mg doses absorb better than one 1000 mg dose.
- Ensure adequate vitamin D. Calcium absorption is vitamin D-dependent. Aim for 25(OH)D above 30 ng/mL. Most older adults benefit from 1000-2000 IU vitamin D3 daily.
- Bone density screening. If you are on long-term PPI therapy, discuss a baseline DEXA scan with your doctor, especially if you have other osteoporosis risk factors (postmenopausal, low body weight, smoking, family history, steroid use).
- Get dietary calcium too. Dairy, fortified plant milks, sardines, kale, and tofu provide calcium that is partially shielded from the absorption issue because it comes complexed with food matrices.
Which specific products are affected?
Calcium carbonate is in Tums, Caltrate 600+D, OsCal, Viactiv, Rolaids, and most calcium-fortified antacids. It is also the cheapest form and the one used in most generic multivitamins. Calcium citrate is in Citracal, Solgar Calcium Citrate, and many "easy to absorb" calcium products. Read the supplement facts panel.
Less common forms: calcium phosphate (Posture-D) is intermediate. Calcium gluconate and calcium lactate are well-absorbed but contain very little elemental calcium per pill, so you would need many tablets to reach 1000 mg.
All PPIs share this effect to varying degrees: omeprazole (Prilosec), esomeprazole (Nexium), lansoprazole (Prevacid), pantoprazole (Protonix), rabeprazole (AcipHex), dexlansoprazole (Dexilant). H2 blockers (famotidine, cimetidine) suppress acid less than PPIs but can have a similar smaller effect.
The bottom line
Omeprazole impairs absorption of calcium carbonate because that form needs stomach acid to dissolve, and chronic PPI use is associated with increased fracture risk per FDA labeling. The simple fix: switch to calcium citrate, which absorbs well in low-acid environments. Take it with food, split doses to 500 mg or less, and pair it with adequate vitamin D. If you have other osteoporosis risk factors and you are on long-term PPI therapy, ask your doctor about a bone density scan. The goal is not to stop the PPI if it is medically necessary, but to neutralize the calcium-absorption side effect with the right form, dose, and timing.