Balanced placebo design with marijuana: pharmacological and expectancy effects on impulsivity and risk taking

Abstract

Rationale: Marijuana is believed to increase impulsivity and risk taking, but the processes whereby it affects such behaviors are not understood. Indeed, either the pharmacologic effect of delta-9-tetrahydrocannabinol (THC) or the expectancy of receiving it may lead to deficits in cognitive processing and increases in risk taking.

Objectives and methods: We examined the relative effects of expecting to receive active marijuana and the pharmacological drug effects using a balanced placebo design. Young adult regular marijuana users (N = 136) were randomly assigned into one of four groups in a two × two instructional set (Told THC vs. Told no THC) by drug administration (smoked marijuana with 2.8 % THC vs. placebo) design. Dependent measures included subjective intoxication, behavioral impulsivity, and decision-making related to risky behaviors.

Results: Active THC, regardless of expectancy, impaired inhibition on the Stop Signal and Stroop Color-Word tasks. Expectancy of having smoked THC, regardless of active drug, decreased impulsive decision-making on a delay discounting task among participants reporting no deception and increased perception of sexual risk among women, consistent with a compensatory effect. Expectancy of smoking THC in combination with active THC increased negative perceptions from risky alcohol use. Active drug and expectancy independently increased subjective intoxication.

Conclusions: Results highlight the importance of marijuana expectancy effects as users believing they are smoking marijuana may compensate for expected intoxication effects when engaged in deliberate decision-making by making less impulsive and risky decisions. Effects of marijuana on impulsive disinhibition, by contrast, reflect direct pharmacologic effects for which participants did not compensate.

Fig. 1

Mean Stop Signal Reaction Time (SSRT) difference scores (baseline subtracted from post-smoking values) by two experimental factors: stimulus expectancy (Told) and drug (Received). Relative to placebo, THC significantly increased SSRT (time to inhibit a prepotent response). Effect of stimulus expectancy manipulation (Told THC vs. Told Placebo) was nonsignificant

Fig. 2

Mean adjusted indifference point difference scores (baseline subtracted from post-smoking values) on the EDT task (ranging from 0.0 (steepest possible discounting) to 1.0 (no discounting)) by two experimental factors: stimulus expectancy (Told) and drug (Received). Relative to placebo, THC did not significantly change delayed discounting. Those in Told THC condition discounted delayed rewards less than those in Told Placebo condition