What are the differences between 500 mg niaspan (niacin extended-release) tablets and niacan you can buy otc?

NiaSpan (niacin). NiaSpan (niacin) is a slow release variety, while the OTC Niacin versions are not - the latter tend to cause facial flushing and redness after use. There's a flush free OTC version which negates this symptom.
Rate of action. niaspan is extended release, where as niacin otc is not. Some literature suggests risk of liver toxicity is lower with the extended release versions. I believe it's possible to obtain otc nicotinic acid. Ask the store .