One of the classes of drugs that causes the biggest dependence reaction, most rapidly, is opioids. When someone takes heroin or a related drug, the amount of opioids in their system is much higher than would ever happen naturally. The brain isn’t used to these huge doses of these chemicals, so it begins to fight back. The neurons that respond to opioids begin to become less sensitive, to counter their effects. This is known as tolerance, and means the brain can function more ‘normally’ when the drug is in the person’s system, but it also has two knock-on effects. First, it means that larger
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