Clen (Clenbuterol) is a powerful stimulant used by athletes to burn off fat. It works by increasing body temperature and heat production. Like all stimulants, Clen can be addictive and cause unwanted side effects such as excessive heart rate, dry mouth, restlessness, insomnia and loss of appetite. Smaller doses are recommended for first-time users and increasing to no more than 200 mcgs/day. Clen should be used in cycles to allow the body to recover and using an antihistamine like Benadryl or Ketotifen during off cycles is recommended.