physical dependence

Physical dependence is an adapted state in which the body has rebuilt itself around the constant presence of a drug, so that taking it away triggers a withdrawal reaction. Imagine leaning steadily against a wall: you adjust your balance to it without noticing, and only when the wall is suddenly removed do you stumble. The body has been compensating all along; withdrawal is that compensation laid bare.

It develops because cells counteract a drug's continued effect. With opioids, neurons that have been chronically dampened ramp up their excitability to fight back; while the drug is present this is invisible, but the moment it is removed the now-overactive system runs unopposed, producing the opposite of the drug's effects — sweating, agitation, cramps, diarrhoea, and craving. Withdrawal symptoms are typically a mirror image of what the drug used to do.

Crucially, physical dependence is not the same as addiction. A patient taking opioids correctly for cancer pain, or a stable patient on certain blood-pressure or antidepressant drugs, can become physically dependent — they would feel withdrawal if the drug stopped abruptly — without ever showing the compulsive, harmful drug-seeking that defines addiction. This is why such drugs are tapered, not stopped suddenly, and why labelling a dependent patient an "addict" is both wrong and harmful.