Adaptation (physiological)

Adaptation is the process by which the body adjusts its own function in response to a repeated stimulus, whether a drug, a stressor or an environmental change.

At the cellular level, repeated exposure to a compound can cause receptors to become less responsive or fewer in number, a process called desensitization or downregulation; the resulting reduced effect is tolerance, and its mirror image on stopping the compound is withdrawal. At the whole-body level, adaptation to stress involves the hypothalamic-pituitary-adrenal axis and cortisol, and adaptation to exercise involves muscle and cardiovascular remodeling. Adaptation is generally protective, but it can also mean a stimulus loses its effect over time.

Cannabinoid receptors adapt readily: regular THC use downregulates CB1 receptors in the brain, which underlies THC tolerance, and the receptors recover after abstinence. Cannabidiol appears to produce little receptor adaptation of this kind in the research so far, though data in people are limited. The endocannabinoid system as a whole is often described as a homeostatic regulator, meaning it participates in how tissues adjust to change.