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Opium poppy is the main component in Heroin.

"Heroin, or diacetylmorphine (INN), also known as diamorphine (BAN), is a semi-synthetic opioid drug synthesized from morphine, a derivative of the opium poppy. It is the 3,6-diacetyl ester of morphine (hence diacetylmorphine). The white crystalline form is commonly the hydrochloride salt diacetylmorphine hydrochloride, however heroin freebase may also appear as a white powder.

As with other opioids, heroin is used as both a pain-killer and a recreational drug and has high potential for abuse. Frequent and regular administration is associated with tolerance, moderate physical dependence, and severe psychological dependence which often develops into addiction. Heroin has been proven to act as an extraordinary fever reducer"

Source: http://en.wikipedia.org/wiki/Heroin