There are too many factors involved to provide an easy answer. The biological make-up of your body, how sensitive you may be to a certain drug and the chemical make-up of the drug itself can all play a role in if or how quickly that you might become addicted.
Some people might be able to use a drug many times without suffering any ill effects, while another person might take the same drug and have a bad reaction or even overdose with first time they use it. Likewise, one person may become addicted to a drug the first time they use it, while another person might never form an addiction.
Just as there are vast differences between the people doing drugs, there are also big differences between the drugs themselves. For example, you may never use enough powdered cocaine to become addicted to it, but you might become addicted to crack cocaine or heroin the first time you try it.
The bottom line is, all drugs can potentially have life-threatening consequences, and individuals can have very different reactions to the same drug. If you are particularly sensitive to the effects of a certain drug, trying it even once could potentially be dangerous.
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Source: National Institute on Drug Abuse. "Frequently Asked Questions." The Science of Drug Abuse and Addiction. Accessed February 2014