Next-Day Cannabis Research Should Not Count Unclear Performance Tests as Negative Findings

A systematic review of next-day THC effects distinguished tests showing no effect from tests with insufficient information. Treating both groups as negative would overstate what the evidence established.

The review evaluated tests within studies

Twenty studies involving 458 participants contributed 345 performance tests. Those are different counts: multiple tests can arise from one study or participant group. The findings should not be presented as 345 independent trials or 345 people.

The unclear category was substantial

The authors classified 209 tests as showing no next-day effect and 121 as unclear because information was insufficient. Twelve indicated impairing effects and three indicated enhancing effects. These classifications preserve uncertainty rather than forcing every test into an impaired-versus-unimpaired choice.

Test sensitivity and study quality also mattered

Some tests without a next-day effect had not demonstrated an acute effect either. A task that fails to detect an acute change supplies a limited basis for strong recovery claims. The review also assessed bias and found important methodological differences between studies.

Interpreting the evidence

The synthesis found limited evidence supporting next-day impairment overall, while calling for better methods. It does not establish that impairment never persists or that every uncertain result is reassuring. A precise account preserves unclear outcomes, test sensitivity, exposure and study quality.

Related reading: interpreting cannabis tolerance claims.

Research reference

Danielle McCartney; Anastasia Suraev; Iain S. McGregor (2023). The “Next Day” Effects of Cannabis Use: A Systematic Review. Cannabis and Cannabinoid Research. doi:10.1089/can.2022.0185.

This discussion draws on the abstract and introductory material in the supplied research synthesis.