Increased platelet aggregation responses to 5‐hydroxytryptamine in patients taking chlorpromazine.

1 The aggregation response of platelets induced by 5-HT was greatly increased in psychiatric patients receiving chlorpromazine therapy when compared with normal volunteers and psychiatric patients not receiving chlorpromazine. 2 Platelet aggregation responses to ADP were normal during chlorpromazine therapy, but 5-HT induced aggregation was increased in rate and the typical transient reversible response was converted to an irreversible response in all subjects. This was usually indistinguishable from the ADP response. 3 When chlorpromazine therapy was stopped, plasma concentrations of chlorpromazine, monodesmethylchlorpromazine and chlorpromazine sulphoxide fell rapidly within one week, whereas 5-HT induced platelet aggregation responses became normal after three weeks. The enhanced responses returned when chlorpromazine therapy was re-instituted. 4 It is possible that platelet aggregation responses to 5-HT in vitro could prove to be a useful index of the pharmacological effect of chlorpromazine in vivo.