The review included 17 studies, involving 639 participants in total. There were 13 RCTs, 5 of which had parallel groups (360 participants) and 8 of which had a crossover design (188 participants). In addition, there were 3 uncontrolled studies (88 participants) and 1 case series (3 participants).
Single dose of secretin (12 studies).
One case series with 3 children reported significant improvements in communication and social behaviour and GI functioning within 5 weeks. One uncontrolled study with 56 children found statistical improvements in several categories of an autism rating scale, but these were not classed as clinically significant. Nine placebo-controlled RCTs found no difference between secretin and placebo in effect on a range of outcomes including autism-related behaviours, communication behaviours, cognitive skills and medical condition. One RCT reported a marginally significant improvement in autistic behaviours with secretin compared with placebo after 3 weeks, but not at 6 weeks.
Multiple doses of secretin (3 studies).
Two RCTs involving children, one giving two doses of secretin or placebo 6 weeks apart and the other giving 6 doses of secretin or placebo over 4 weeks, both found no significant differences between the two groups in effects on autistic behaviours, language or cognitive functioning. One uncontrolled study of adults found no significant effect of homeopathic secretin given twice daily for 12 weeks.
Effects on children with GI problems (2 studies).
One RCT involving 19 children found significant effects of secretin, compared with placebo, on some autism behaviours only for the subgroup of participants with chronic diarrhoea. An uncontrolled study of 20 children with autism and GI symptoms found no significant effect of secretin on autistic and social behaviours.