The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.
Rarely, anaphylactic shock (severe allergic reaction) has been described after intravenous or oral niacin therapy.
Side Effects and Warnings
Most people taking niacin experience skin flushing and a warm sensation, especially of the face, neck, and ears when they begin treatment or increase dose. This reaction is usually mild, but has been intolerable enough to cause up to half of participants in studies to stop therapy. Dry skin and itching is also commonly experienced. Taking aspirin or non-steroidal anti-inflammatory drugs such as ibuprofen (Advil®, Motrin®), naproxen (Naprosyn®), or indomethacin (Indocin®) can reduce the flushing. Use of an antihistamine 15 minutes prior to a niacin dose may also be helpful. Slow-release niacin products may have less skin flushing than regular release niacin preparations or may simply delay the appearance of flushing. The flushing response often decreases on its own after one to two weeks of therapy. Mild stomach upset, nausea, vomiting, and diarrhea also may occur when beginning niacin therapy; these symptoms usually resolve with continued use.
More serious side effects include liver toxicity, worsening of stomach ulcers, and altered blood sugar or insulin levels or uric acid concentrations. Numerous case reports describe liver toxicity, including increased liver enzyme levels in the blood, skin yellowing (jaundice), fluid in the abdomen (ascites), or liver failure. Monitoring of liver blood tests while using niacin is recommended. While slow-release niacin products may have less skin flushing than regular release niacin preparations, they may worsen stomach and liver side effects. High doses of niacin may also cause low blood pressure.
Lactic acidosis, muscle cell damage (myopathy), and increased blood levels of creatine kinase (a marker of muscle damage) have been reported in studies.
Abnormal heart rhythms, heart palpitations, and circulatory collapse (when injected through the veins) have occurred following niacin use. Based on human research, taking niacin alone or with colestipol may increase blood homocysteine levels. High levels of homocysteine have been associated with an increased risk of heart disease.
Blood clotting problems have been reported during treatment with sustained-release niacin. Low white blood cell number (leukopenia) and slightly increased blood eosinophils have also been reported.
Rarely reported side effects include headache, dry eye, tooth or gum pain, dizziness, breathing difficulty, increased anxiety, panic attacks, rash, and decreased thyroid function (hypothyroidism). There are published accounts of temporary side effects of the eye including macular swelling and blurred vision as well as toxic amblyopia ("lazy eye"). These side effects resolved when niacin was stopped.
Pregnancy and Breastfeeding
Use of niacin supplementation during pregnancy or breastfeeding is not recommended due to lack of sufficient research of safety and effectiveness.