One dose of heroin will not make you an addict. An estimated 25 to 40% of street users are not physically dependent. There are plenty of regular users who set strict limits on their heroin-use and do not become addicts.

However, heroin is highly physically addictive, more so than morphine, and on a par with nicotine. Regularity of use is as important as the amount in the development of addiction.

To compound things, the body quickly builds a tolerance to the drug. Long term, larger and larger doses are needed to achieve the same effect.

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where you take it
Bizarrely, tolerance also appears to be partially related to the environment where the drug is taken. Studies have shown that users who take heroin in an unusual or different place to normal are more likely to overdose.

No-one knows why this should be, but one theory is that the bodies of addicts who go to the same location to take the drug every time become conditioned to taking it there: their body gears up for what's to come and therefore they have a higher tolerance in that location. When they shoot up somewhere else, their body hasn't prepared itself and they overdose. This, however, is only speculation.