If you are currently being treated with any of the following medications, you should not use calcium supplements without first talking to your healthcare provider

Alendronate
Calcium may interfere with the absorption of alendronate, a medication used to treat osteoporosis. Calcium containing products, therefore, should be taken at least two hours before or after alendronate.

Antacids, Aluminum-containing
When calcium citrate is taken with aluminum containing antacids, the amount of aluminum absorbed into the blood stream may be increased significantly. This is a particular problem for people with kidney disease in whom the aluminum levels may become toxic. In addition, aluminum-containing antacids may increase the loss of calcium in the urine.

Blood Pressure Medications
Taking calcium with a beta-blocker (such as atenolol), a group of medications used for the treatment of high blood pressure or heart disorders, may interfere with blood levels of both the calcium and the beta-blocker. Study results are conflicting, however. Until more is known, individuals taking atenolol, or another beta blocker, should have their blood pressure checked before and after adding calcium supplements or calcium containing antacids to their medication regimen.

Similarly, it has been reported that calcium may reverse the therapeutic effects as well as the side effects of calcium channel blockers (such as verapamil) often prescribed for the treatment of high blood pressure. These study results are also controversial. People taking verapamil or another calcium channel blocker along with calcium supplements should likely have their blood pressure checked regularly.

Cholesterol-lowering Medications
A class of medications known as bile acid sequestrants (including cholestyramine, colestipol, and colesevelam), used to treat high cholesterol, may interfere with normal calcium absorption and increase the loss of calcium in the urine. Supplementation, therefore, with calcium and vitamin D may be recommended by your healthcare provider.

Corticosteroids
Corticosteroid medications reduce the absorption of calcium, thereby increasing the risk for bone loss and osteoporosis over time. This is of particular concern for anyone who is maintained on long-term steroids.

Digoxin
High levels of calcium may increase the likelihood of a toxic reaction to digoxin, a medication used to treat irregular heart rhythms. On the other hand, low levels of calcium cause this medication to be ineffective. People who are taking digoxin should have calcium levels monitored in the blood closely.

Diuretics
Two different classes of diuretics interact with calcium in opposite ways—thiazide diuretics such as hydrochlorothiazide can raise calcium levels in the blood, while loop diuretics, such as furosemide and bumetanide, can decrease calcium levels. In addition, amiloride, a potassium-sparing diuretic, may decrease the amount of calcium excreted in the urine (and subsequently increase calcium levels in the blood), especially in people with kidney stones.

Estrogens
Estrogens may contribute to an overall increase in calcium blood levels. Taking calcium supplements together with estrogens improves gain in bone density significantly.

Gentamicin
Taking calcium during treatment with the antibiotic gentamicin may increase the potential for toxic effects on the kidneys.

Metformin
Metformin, a medication used to treat type 2 diabetes, can deplete levels of vitamin B12. Some early evidence suggests that calcium supplements may prevent or eliminate this negative effect of metformin. More research is needed.

Antibiotics, Quinolones
Calcium can interfere with the body's ability to absorb quinolone antibiotics (such as ciprofloxacin, levofloxacin, norfloxacin, and ofloxacin). If taking calcium containing supplements or antacids, therefore, you should take them two to four hours before or after taking quinolone antibiotics.

Seizure Medications
Low levels of calcium have been reported with high doses of seizure medications, such as phenytoin, which may decrease calcium absorption. Some physicians recommend vitamin D along with anti-seizure drugs to try to prevent the development of low calcium levels.

Tetracyclines
Calcium can interfere with the body's ability to absorb tetracycline medications (including doxycycline, minocycline, and tetracycline) and, therefore, diminish their effectiveness. Calcium containing supplements and antacids should be taken at least two hours before or after taking these drugs.