Introduction and Aims. A large legal market for party pills containing benzylpiperazine (BZP) and trifluoromethylphenylpiperazine (TFMPP) developed in New Zealand after 2004. The use of these party pills has been associated with adverse health effects. The purpose of this paper was to assess a general population sample of party pill users to investigate the relationship between (1) patterns of use of BZP/TFMPP party pills and concurrent use of other drug types, and (2) adverse side effects from BZP/TFMPP party pill use. Design. A national household survey of the use of BZP/TFMPP party pills was conducted using a computer-assisted telephone interviewing (CATI) facility. The quantity of BZP and TFMPP in each brand of party pill was obtained from the National Poisons Centre. Multiple logistic regression analysis was used to identify independent predictors of having experienced adverse side effects from party pills. Results. The mean quantity of BZP/TFMPP taken on an occasion of greatest use was 533 mg (median 400 mg, range 43 – 2500 mg). Being female, using cannabis and other drugs concurrently with BZP/TFMPP party pills, taking large quantities of party pills in a single session and taking 5-hydroxytryptophan (5-HTP) recovery pills at the same time as party pills were independent predictors of having experienced an adverse problem from party pills. Conclusions. Females may be at greater risk of experiencing problems from BZP/TFMPP party pills due to their smaller physical size. Taking 5-HTP ‘recovery’ pills with party pills may increase the risk of adverse effects as both substances increase users' levels of serotonin.