Compliance with medication regimens and clinical trial schedules was evaluated during a study of vigabatrin (VGB), an antiepileptic drug (AED). Medication Event Monitors (MEMS, Aprex Corp., Fremont, CA, U.S.A.) were provided to monitor use of VGB and other AEDs administered to 111 patients at 10 sites. MEMS reports showed the number of doses administered daily, times of doses, and intervals between doses. The 66 patients whose data were evaluable took VGB as prescribed (twice daily, b.i.d.) on 89 +/- 7% of days in the clinical trial (mean 189 +/- 63 days). However, only 66 +/- 24% of doses were taken within the 9-15-h dose interval window for twice-daily dosing, a lower rate than that for dose frequency compliance (p < 0.001). Concomitant medications prescribed b.i.d. (n = 66) (86 +/- 11% dose frequency compliance) were taken at lower rates than VGB (p < 0.02). Interval compliance also was lower for concomitant b.i.d. medications (59 +/- 26%) than for VGB (p < 0.01). Dose frequency compliance for thrice-daily (t.i.d.) medications (n = 36) was 80 +/- 18 and 40 +/- 19% for interval compliance (6-10 h) (both p < 0.0001 vs. VGB). Dose frequency compliance for four times daily (q.i.d.) medications (n = 23) was 80 +/- 23 and 33 +/- 18% for interval compliance (4-8 h) (both p < 0.0001 vs. VGB). Patients at eight sites did not use MEMS properly, often for practical reasons, voiding including of data for 93 medications (32%) because of noncompliance with the study design to monitor compliance.(ABSTRACT TRUNCATED AT 250 WORDS)