A double blind-cross-over randomised clinical trial has been conducted to compare the antiemetic effects of tetrahydrocannabinol, thiethylperazine and metoclopramide. There were no significant differences in the antiemetic effects of these drugs. The incidence of adverse reactions as recorded by both the staff and the patients was significantly higher in the tetrahydrocannabinol group than in either the metoclopramide or thiethylperazine groups. This trial has established that in the dosages used tetrahydrocannabinol given by mouth has an antiemetic effect of approximately the same order as thiethylperazine and metoclopramide. However, its adverse effects are sufficiently greater than those of the other agents to prevent is widespread usage for this purpose. Tetrahydrocannabinol taken by mouth is not recommended as a routine antiemetic agent in cancer chemotherapy.