Common side effects of vitamin B-3, also known as niacin, include flushing, dizziness, upset stomach, intestinal gas and mouth pain, according to WebMD. The flushing reaction can cause redness, tingling and itching of the face, chest and arms, along with a headache. Drinking alcohol can worsen the flushing.
Flushing reactions usually dissipate when the body gets used to vitamin B-3, notes WebMD. Taking a 325 milligram dose of aspirin prior to taking vitamin B-3 helps reduce persistent flushing.
Large doses of more than 3 grams per day of vitamin B-3 can cause more serious side effects, such as ulcers of the digestive tract, liver problems, irregular heartbeat, high blood sugar and loss of vision, states WebMD.