A person's first dose of heroin typically makes them feel very ill. They may feel dizzy or lightheaded, or even vomit. But after this initial unpleasantness, most drug users will find taking opiates an enjoyable experience. A hit will make the user feel warm, happy or relaxed and boost their feeling of well being. This impact on the neural pathways of the brain can cause changes very quickly, so that person becomes dependent on the drug after just a few instances of taking it.
Because opium has such a powerful effect on the brain, the neural pathways can re-wire themselves in response to the drug. This causes the person to develop a tolerance to the opium, and they will need to increase how much they use if they want to elicit the same feelings of well being that they used to. This can become very dangerous.
If a person with a regular habit stops taking the drug for a few days, their tolerance will drop very rapidly. Consequently, when the person next takes their "usual" hit the brain cannot cope with the level of opium and an overdose occurs. An overdose can leave the person in a coma, or even lead to death.
Injecting a drug carries its own risks too. Over time, substance abuse will damage the arteries and veins and lead to infections or gangrene. Sharing needles also exposes a drug addict to the risk of hepatitis C and AIDs.
Because of the high risk associated with relapses and overdose, a person should always seek help from a doctor or treatment center in order to overcome their addiction.