Asked by: Carel Lucas, Perth, Australia

Addictive drugs interfere with neurotransmitters, the chemicals that transmit signals around the brain, and their receptors. This causes changes to the brain’s reward system. This can create craving and tolerance, so that a higher dose is needed to have the same effect. Some also have unpleasant withdrawal symptoms that are only relieved by taking more.

Heroin is the most addictive drug known. It mimics the brain’s own endorphins, the natural opioids that induce pleasure and reduce pain. The brain responds by reducing the number and sensitivity of its opioid receptors so that more of the drug is needed. Nicotine increases dopamine and activates the brain’s reward pathways. Again, the brain compensates so that more nicotine is required.

People can also become addicted to activities such as sex, gambling or shopping, but the mechanisms underlying these addictions are less well understood.

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