If you are currently being treated with any of the following medications, you should not use vitamin D supplements without first talking to your health care provider.

Vitamin D levels may be increased by the following medications:

Estrogen -- Hormone replacement therapy with estrogen appears to increase vitamin D levels in the blood; this may have a beneficial effect on calcium and bone metabolism. In addition, use of vitamin D supplements in conjunction with estrogen replacement therapy (ERT) increases bone mass more than ERT alone. However, this benefit may be lost with the addition of progesterone.

Isoniazid (INH) -- INH, a medication used to treat tuberculosis, may raise blood levels of vitamin D.

Thiazide -- Diuretics in this class (such as hydrochlorothiazide) increase the activity of vitamin D and can lead to inappropriately high calcium levels in the blood.

Vitamin D levels may be decreased, or its absorption may be reduced, by the following medications:

Antacids -- Taking certain antacids for long periods of time may alter the levels, metabolism, and availability of vitamin D.

Calcium channel blockers (such as verapamil ) -- These medications, used to treat high blood pressure and heart conditions, may decrease the production of vitamin D by the body.

Cholestyramine -- This cholesterol-lowering medication, known as a bile acid sequestrant, interferes with the absorption of vitamin D (as well as other fat-soluble vitamins).

Phenobarbital, phenytoin, and other anticonvulsant medications -- These medications may accelerate the body's use of vitamin D.

Mineral oil -- Mineral oil also interferes with absorption.

In addition, Vitamin D may enhance the effects of doxorubicin, a medicine used to treat a variety of cancers. More research is needed.

Some doctors recommend following calcium levels closely if vitamin D is taken with digoxin, a medication used to treat irregular heart rhythms. This is because vitamin D improves absorption of calcium. Calcium, in turn, can increase the likelihood of a toxic reaction from this medication.

Weight loss products -- Orlistat (also known as alli), a medication used for weight loss, and olestra, a substance added to certain food products, are both intended to bind to fat and prevent the absorption of fat and the associated calories. Because of their effects on fat, orlistat and olestra may also prevent the absorption of fat-soluble vitamins such as vitamin D. Given this concern and possibility, the Food and Drug Administration now requires that vitamin D and other fat-soluble vitamins (namely, A, E, and K) be added to food products containing olestra. How well the body absorbs and uses vitamin D from such food products is not clear. In addition, physicians who prescribe orlistat add a multivitamin with fat soluble vitamins to the regimen.