An orphan drug is any drug developed under the 1983 U.S. Orphan Drug Act, which concerns drugs for orphan diseases, namely such affecting less than 200,000 people in the US. Developing a drug for groups that small would be financially unsound. Therefore, development of drugs for such diseases is rewarded by tax reductions and a monopoly for that drug for a limited time (7 years). Under that premise, many drugs have been developed, among them such against AIDS, cystic fibrosis, and snake venom.
