Cannabinoid medication might be useful in the treatment of the symptoms in patients with Tourette's syndrome. At the present time only two relevant studies have been conducted. Both studies used tetrahydrocannabinol (Δ9THC). In both studies Δ9THC was associated with tic reduction. However the sample size was small and a large number of multiple comparisons were made . There were only 28 participants in total, since eight participants took part in both studies. Possibly the patients who derived the greatest benefit and experienced the least adverse effects would be the most inclined to participate in further studies. There were a high number of drop outs/exclusions in the six week study and it is unclear whether intention to treat analysis (ITT) was performed. The results that are reported are analyses done on the patients who remained in the study on the study medication at the correct dose. In reality, patients do opt not to continue treatment if there is limited efficacy or unpalatable side effects. This introduces attrition bias. Whilst there were some significant results, the authors themselves accept that very few of these results are significant if a Bonferroni correction is performed. It is possible that cannabinoid medication has a beneficial effect which is too weak to be detected using ITT and such a small sample size. There is some weak evidence that cannabinoid medication may have an effect on obsessive compulsive behaviour but the measure used was an addition to the TSSL which has not been validated.There were no data on the effect of Δ9THC on quality of life.There is not enough evidence to support the use of cannabinoids in treating tics and obsessive compulsive behaviour in people with Tourette's syndrome.