Physical fatigue

Physical fatigue is a sense of profound bodily tiredness and reduced capacity for physical effort.

As a drug effect it is acute and dose-proportional, easing as the substance wears off, distinguishing it from chronic fatigue conditions. It ranges from a mild heaviness in the limbs to a near-inability to remain upright or complete simple movements.

Physical fatigue is a frequently reported interoceptive effect across psychoactive substances. It is most commonly induced at moderate to high doses under deliriants such as diphenhydramine and datura, and under psychedelics such as LSD and psilocybin mushrooms. It also appears under nootropics such as 5-hydroxytryptophan and galantamine, and under depressants such as clonidine.

Physical fatigue is felt as a pervasive heaviness throughout the body: muscles feel laden, joints stiff, and lifting a limb demands effort disproportionate to the task. It commonly comes with drooping eyelids, a slackening of posture, and a dull ache that settles across the shoulders and legs.

Attending to the exhaustion tends to deepen it — the awareness of effort required makes further movement feel less worthwhile. At higher doses, particularly under deliriants, it can become so total that standing or walking becomes difficult.

Physical fatigue arises through different pathways depending on the drug class:

• Deliriants — block muscarinic acetylcholine receptors, disrupting neuromuscular coordination and suppressing wakefulness-promoting signals.
• Psychedelics — act on serotonergic circuits in ways that can leave the body feeling depleted, particularly after prolonged or intense experiences.
• Nootropics — act through serotonergic or cholinergic mechanisms that can produce fatigue as a secondary effect, particularly at moderate to high doses.
• Depressants — suppress central nervous system activity broadly, reducing arousal and voluntary motor tone.

How much each pathway contributes in a given case is not fully understood.

Who experiences it, and how strongly, varies widely. Higher doses raise both likelihood and intensity across all drug classes. Set and setting play a secondary role — physical exertion before or during a session amplifies it, as does poor prior sleep.

Disposition matters too: people with lower baseline energy or chronic health conditions tend to feel it more acutely. Expectation has modest influence — those who anticipate sedation often notice fatigue earlier. Adequate rest beforehand is the most reliable protective factor.