How easily someone gets addicted to drugs depends on various things. A lot of people, who use drugs for the first time, do that to experiment, out of curiosity, or because they are looking for a certain feeling. Whether someone continues to use drugs after that first time depends, among others things, on the effects of the drugs and the susceptibility of the person to them. Also the motives for using drugs are involved; someone who uses because he feels bad runs more risk of becoming a problematic user than someone who uses and feels good.
There is no drug that leads to addiction when only used once. Therefore, it is not possible to maliciously make another person addicted, for instance, by putting drugs in his/her drink. It all depends on the kind of drugs and the kind of person who gets in touch with them. Some drugs lead to more addiction problems, like social problems and criminal activities, than other drugs. Heroin, for example, is very addictive. Cocaine and amphetamines are less addictive, and XTC, hashish and marijuana even less. Crack cocaine is more addictive than cocaine alone. Nevertheless, it is true that someone who is addicted to, say, XTC is more likely to get in contact with dealers and users of heroin, for example.