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

Antibiotics -- Antibiotics, particularly a class known as cephalosporins, reduce the absorption of vitamin K in the body. Long-term use (more than 10 days) of antibiotics may result in vitamin K deficiency because these drugs kill not only harmful bacteria but also beneficial, vitamin K-activating bacteria. This is mot likely to occur in people who already have low levels of vitamin K or are at risk for deficiency (such as those who are malnourished, elderly, or taking warfarin). Cephalosporins include:

• Cefamandole (Mandol)
• Cefoperazone (Cefobid)
• Cefmetazole (Zefazone)
• Cefotetan (Cefotan)

Phenytoin (Dilantin) -- Phenytoin interferes with the body's ability to use vitamin K. Taking anticonvulsants (such as phenytoin) during pregnancy or while breastfeeding may deplete vitamin K in newborns.

Warfarin (Coumadin) -- Vitamin K reduces the effects of the blood-thinning medication warfarin, rendering the medication ineffective. Vitamin K should not be taken while taking warfarin, and foods containing high amounts of vitamin K should be avoided.

Orlistat (Xenical, alli) and Olestra -- Orlistat, a medication used for weight loss, and olestra, a substance added to certain food products, prevent the absorption of fat and can reduce the body's absorption of fat-soluble vitamins. The Food and Drug Administration now requires that vitamin K and other fat-soluble vitamins (A, D, and E) be added to food products containing olestra. In addition, physicians who prescribe orlistat add a multivitamin with fat soluble vitamins to the regimen.

The fact that vitamin K is now added to olestra-containing foods is important to know if you should not be taking vitamin K (if you are on the blood thinner warfarin, for example).

Bile acid sequestrants -- These medications, used to reduce cholesterol, reduce the overall absorption of dietary fats and may also reduce absorption of fat-soluble vitamins. If you take one of these drugs, your doctor may recommend a vitamin K supplement:

• Cholestyramine (Questran)
• Colestipol (Colestid)
• Colsevelam (Welchol)