Cold-Water Stress Did Not Raise Plasma THC in a Small Human Study

A brief cold-water stressor did not significantly increase plasma THC in a small study of regular cannabis users. The result tests one version of the “reintoxication” hypothesis; it does not establish what happens after every severe or prolonged stressor.

Stress and fat breakdown were measured

Fifteen participants completed the analyzed protocol, involving ten minutes of immersion in water at approximately 10°C. They reported cannabis use on an average of five days per week and were instructed to abstain overnight and arrive fasting.

Researchers collected plasma before immersion, shortly afterward and two hours later. They also measured free fatty acids and glycerol as indices of lipolysis, physiological stress, subjective drug effects and performance on three cognitive or psychomotor tasks.

Lipolysis did not produce the predicted THC increase

Heart rate and systolic blood pressure increased, calmness decreased, and lipid-related markers rose modestly. Nevertheless, plasma THC showed no significant time effect (p = 0.950). THC-COOH did not increase immediately after immersion and was lower at the later assessment.

The cognitive and psychomotor measures showed no significant time effects. Small increases in ratings of feeling stoned and drug effect were statistically significant, but the absolute ratings were very low. Calling every subjective measure unchanged would therefore misstate the findings.

The experiment does not represent every traumatic event

This was a single-arm before-and-after study without a randomized control condition or blinding. Fasting and the passage of time could have contributed to some observed changes. Blood was sampled after, rather than continuously during, immersion, limiting what can be said about a very brief intervening fluctuation.

The modest stressor, small sample and participants’ use histories also limit extrapolation to a crash, assault, extreme exercise or much heavier exposure. The study supplies no numerical correction for a case blood result. Its supported conclusion is narrower: measured lipolysis after this cold-water protocol was not accompanied by the hypothesized plasma THC rise or a detected task deficit.

Related: interpreting THC concentrations separately from impairment.

Primary research

Does acute stress induced via cold water immersion increase blood THC concentrations in regular cannabis users?. DOI: 10.1007/s00213-025-06833-8.