Objective  To compare the effectiveness of vaginal misoprostol administered 6 or 12 hourly for second trimester pregnancy termination.

Design  A randomised controlled trial.

Setting  University teaching hospital.

Sample  Two hundred and seventy-nine pregnant women at gestations between 14 and 26 weeks undergoing pregnancy termination.

Methods  Women were randomised to receive 600-μg misoprostol tablets vaginally either every 6 hours or every 12 hours until abortion occurred.

Main outcome measures  Induction–abortion interval, success rate within 24 and 48 hours and adverse effects.

Results  There was no significant difference in the median induction to abortion interval 6 hours (16 hours) and 12 hours (16 hours; P= 0.80). The total dose of misoprostol was higher in the 6-hour group (1800 vs 1200 μg). The cumulative abortion rates within 24 hours were 74% and 67% and within 48 hours 94% and 92%, in the 6- and 12-hour groups, respectively. Fever was more common in the 6-hour group (53%) versus the 12-hour group (31%; P < 0.001). The incidence of nausea, vomiting, diarrhoea, severe bleeding and abdominal pain were similar.

Conclusions  Misoprostol (600 μg) administered at 12-hour intervals was associated with fewer adverse effects and was as effective as a 6-hour interval.