Q. My doctor gave me a prescription for niacin to lower my cholesterol. Since niacin is a vitamin, can't I just buy it at the health food store?

A. Never substitute over-the-counter niacin for prescription niacin. Niacin is a B vitamin found in meat, fish, milk, eggs, green vegetables and cereals. It is sold in two forms: nicotinic acid and nicotinamide. When taken in low doses (20 to 40 mg) for nutritional purposes, these two forms have identical effects. However, at high doses, each has its own effect.

Over-the-counter niacin supplements sold in health food, grocery and drug stores are in the form of nicotinamide. The type found in prescription niacin is nicotinic acid. In the 1950s, it was shown that large doses of nicotinic acid (but not nicotinamide) could lower blood cholesterol.

Nicotinic acid lowers cholesterol in a different way than the widely prescribed statin medications. Statins mainly lower LDL cholesterol, while niacin dramatically raises HDL. HDL is often referred to as "good cholesterol" because it helps remove LDL cholesterol (the type that causes heart attacks) from the body. HDL is like a garbage truck, picking up stray LDL and sending it to the liver for disposal. Niacin is the most effective medication for increasing HDL; therefore if your HDL levels are too low, your physician may prescribe niacin.

Nicotinic acid used to be available only in an immediate-release form. This meant the whole dose entered the blood stream at once. High doses caused flushing of the skin (redness, tingling and itching). Although this side effect was harmless, it was still annoying. Now, an extended-release version is available by prescription. The extended-release form reduces flushing by approximately 80 percent.

As with other cholesterol-lowering medications, there is a possibility of liver damage when taking large doses of niacin. It also may affect the blood sugar in some people. For these reasons, your physician should monitor your liver function and blood sugar levels while on high-dose niacin therapy.

Dosage for lowering cholesterol is usually 250 to 2,000 milligrams per day. This is quite a bit higher than the 14 to 16 milligrams per day that is recommended to meet our nutritional needs. Because the doses needed to lower cholesterol are much higher than the amount needed for nutrition, and the risk for side effects greater, prescription niacin is considered to be drug therapy rather than nutritional therapy.

Amy Tousman is a registered dietitian and certified diabetes educator with Nutrition Unlimited in Kailua. Hawai'i experts in traditional medicine, naturopathic medicine and diet take turns writing the Prescriptions column. Send your questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; fax 535-8170; e-mail islandlife@honoluluadvertiser.com. This column is not intended to provide medical advice.