Medication adherence mediates the impact of sustained cannabis use on symptom levels in first-episode psychosis.

Both medication non-adherence and co-morbid cannabis abuse are associated with poor clinical outcome in first episode psychosis (FEP). The nature of interaction between adherence to medication and continued cannabis use remains unexplored. The objectives of this study were to examine variation in medication adherence associated with cessation or continuation of cannabis use, and to determine the impact of interaction between cannabis use and adherence to medication on symptom outcome at 12 months. From a consecutive patient cohort (N=192) with a DSM-IV diagnosis of a FEP, 62 patients who met DSM-IV criteria for a currently active cannabis abuse disorder were followed up for one year. Complete data on repeated measures of medication adherence, symptoms, and cannabis use were available for 48 of the 62 patients. Twenty-eight patients (58.7%) continued while 20 (41.2%) stopped cannabis use after entering treatment. While both groups were relatively non-adherent at six months, 25/28 (92%) of the former group became adherent compared to 8/20 (40%) of those who stopped cannabis use (p<.01). While there was no overall effect of continued cannabis use on symptom levels at 12 months, after controlling for medication adherence patients with continuous cannabis use had significantly higher level of symptoms (F(1,30)=2.74, p=0.03). FEP patients with an active cannabis use disorder may make a choice of either stopping cannabis and not taking medications or continuing cannabis but becoming more adherent to medications, adherence to medication appears to help both groups but continuous users remain at higher risk of poor symptom outcome even while on medication.