Rationale: As central 5-hydroxytryptamine (5-HT) is attenuated for a period following a single dose of MDMA ("ecstasy") and low 5-HT is associated with aggression, then MDMA users may be more aggressive in the days following an acute dose of the drug.

Objective: This study therefore aimed to determine if acute use of MDMA is associated with aggression 4 and 7 days later.

Methods: Twenty-nine MDMA users and 32 controls were compared on self-rated aggression and depression on the night of drug use (day 0), 4 and 7 days later. On day 4, participants performed an interpretative bias task in which they processed ambiguous sentences that could be interpreted in either an aggressive or neutral way (e.g. "The painter drew the knife").

Results: MDMA users had faster response times in completing ambiguous aggressive sentences than neutral sentences; controls showed the opposite pattern of performance. In a subsequent recognition task, MDMA users were more confident in judging, and responded faster to, aggressive than neutral sentences; controls again showed the opposite pattern of effects. The level of aggressive interpretative bias positively correlated with extent of MDMA use. Midweek, MDMA users had higher self-rated aggression and depression scores than controls; on day 7, scores of both groups were similar.

Conclusions: MDMA users display a cognitive bias towards interpreting ambiguous information in an aggressive way a few days after taking the drug. Self-rated mid-week low mood and mid-week aggression do not persist 7 days after use of the drug. This pattern of results is consistent both with the acute and residual effects of MDMA on central 5-HT and with the notion that 5-HT plays a role in modulating human aggression.