No one knows how many times a person can use ecstasy before becoming addicted or who's most vulnerable to ecstasy addiction. Genetic makeup, living environment, and other factors probably play a role in a person's susceptibility to addiction to any drug.

560,000 people used heroin last year.

Use of heroin in pregnancy also may increase the risk of a variety of birth defects. What is not entirely clear is whether these effects are directly due to the drug itself or related to the poor health behaviors that women who take heroin often have. It may also be caused by the substances that the heroin often is mixed with when it is made.

Cocaine hydrochloride, the form in which cocaine is snorted or injected is often "cut," or mixed, with things that look like it. The final product can be from 1% to 95% pure. Common additives are sugars, such as mannitol, lactose, or glucose, or even sugar substitutes, and local anesthetics such as tetracaine, procaine, and lidocaine. Quinine, talc, and cornstarch have also been used. Some consumers may unknowingly purchase a supply without any cocaine, but just a cocaine substitute such as caffeine, amphetamine, PCP, procaine, and lidocaine.