The question of carcinogenic effects of hydrazine

In a number of older studies by various authors, hydrazine showed carcinogenic effects after toxic or subtoxic doses. Our own preliminary studies with maximum tolerable doses (rats, s.c. or i.t.), however, yielded negative results, as did the administration of maximum tolerable hydrazine doses simultaneously with benzo(a)pyrene treatment (rat i.t. and mouse i.p.). These preliminary experiments were therefore followed by a carcinogenicity study in rats, in which hydrazine was administered with the drinking water in concentrations of 0 mg/l (= control), 2, 10, and 50 mg/l. Each group comprised 50 male and 50 female Wistar rats. The investigation was continued until the spontaneous death of all the rats. The concentration of 2 mg/l was tolerated with hardly any damage; 10 mg/l proved to be the maximum tolerable dose, and 50 mg/l was clearly toxic although the survival times were not affected appreciably. Only under the toxicity of the highest concentration could a weak carcinogenic effect of hydrazine be detected, namely in the very late occurrence of small, usually benign liver cell tumours (overall 11.5%). A similar study carried out in mice revealed no macroscopic evidence of a carcinogenic effect of hydrazine (histology currently in preparation). According to these results, hydrazine is only weakly carcinogenic even after lifelong ingestion of unequivocally toxic doses. The classification of hydrazine as a "carcinogen" should therefore be re-evaluated.