Also known as vitamin B-3, niacin is an essential vitamin found in foods such as fish, beets and fortified cereals, but it is also commonly available as a dietary supplement or prescription medication. All B vitamins help convert food into energy, but high doses of niacin may be beneficial for a variety of conditions such as high cholesterol. Over-the-counter niacin capsules in 500-mg doses are commonly available, but it is important to speak with a doctor before use to avoid potentially harmful side effects.
Serious Effects
Seek medical attention or call a doctor if any of the following serious side effects occur while taking niacin: fainting or lightheadedness, irregular heartbeats, shortness of breath, yellowing of the skin or whites of the eyes, swelling or muscle pain, weakness with fever and dark-colored urine.
Niacin Flush
According to the University of Maryland Medical Center, high doses -- greater than 50 mg -- of niacin can cause side effects, with the most common being niacin flush. This reaction is characterized by redness of the face, neck and chest, as well as a burning or tingling feeling. This reaction generally goes away after a few weeks. Aspirin or other nonsteroidal anti-inflammatory drugs taken 30 minutes before niacin administration may help reduce the symptoms of niacin flush.
Liver Toxicity
Consuming large doses -- 3 g or more daily -- of niacin may damage the liver and has been reported in 2 to 3 percent of individuals, according to Drugs.com. Individuals with a history of liver disease, alcohol abuse and sulfonylurea use may be at an increased risk of liver damage from niacin use. Persistent fatigue, weight loss and nausea may be indicators of liver toxicity.
FDA Warning
Niacin is classified by the U.S. Food and Drug Administration as a Pregnancy Category C medication. Medications in this category may be harmful to the unborn fetus or to a nursing baby, according to the website SafeFetus.com. Pregnant and nursing mothers should not take niacin unless the benefits outweigh the risks as determined by a trained healthcare provider. Women who become pregnant should stop taking niacin and speak with a doctor.
Minor Effects
Less serious side effects include nausea, diarrhea, bloating, minor muscle cramps or pain, sweating, chills, sleep difficulties and mild dizziness.
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