With the continued use of a physically addictive drug, tolerance develops; i.e., constantly increasing amounts of the drug are needed to duplicate the initial effect. Sharing hypodermic needles used to inject some drugs dramatically increases the risk of contracting AIDS and some types of hepatitis. In addition, increased sexual activity among drug users, both in prostitution and from the disinhibiting effect of some drugs, also puts them at a higher risk of AIDS and other sexually transmitted diseases