Opium is the dried latex extracted from the capsules of Papaver Somniferum, cultivated in many countries for countless centuries. It contains a number of alkaloids, the most important of which is morphine. In 1890 a pharmaceutical company, Bayer and Co., started using morphine as an effective painkiller with no severe side effects.

Only small quantities of morphine can cross the haematoencephalic barrier, which is more easily reached by heroin; this fact may explain the flash experienced by a drug addict after an intravenous injection. Repeated use of heroin creates a tolerance. Opioids have an anesthetizing effect on the nervous system. The endocrine system shows diminished levels of LH, which implies a drop of testosterone in males, a drop of FSH, and a rise of prolactin and GH levels in females, leading to amenorrhea and infertility. The bulbar centres of the respiratory system become depressed; such depression is particularly severe in case of overdose. As to the vascular system, patients suffer from orthostatic hypotension.

Signs and symptoms of withdrawal from heroin appear 8-12 hours after the last dose: sweating, rhinorrhea, lacrimation, yawns, agitation, restlessness (achatisia), and later mydriasis, horripilation, trembling, fluster, abdominal pain with vomit and diarrhoea, muscular pain (mostly in the lumbar region), tachycardia and higher arterial pressure. Withdrawal symptoms peak 2-3 days after the last dose of heroin, and generally subside within 4-6 days. Continuous withdrawal symptoms also include behavioural changes which can last for some months, such as uneasiness, anxiety, depression, asthenia, premature ejaculation and insomnia. Methadone and buprenorphine withdrawal symptoms last longer, as such substances have a longer half-life than heroin.