Immediate-release niacin enters the bloodstream right away. Sustained-release niacin enters the bloodstream gradually. Both immediate-release and sustained-release niacin supplements are available over the counter in 2015, according to About.com.
Extended-release niacin supplements are available only by prescription. Marketed under the trade name Niaspan, extended-release niacin does not injure the liver the way immediate-release and sustained-release niacin can, according to About.com.
Niacin is also called nicotinic acid. It is a B vitamin found in many foods, including green vegetables, fish, meat, yeast and cereal grains. Niacin lowers high cholesterol and reduces blood lipid levels. Niacin may be less effective on people who are severely overweight, says Mayo Clinic.