Guaiacol is a compound used as expectorant. In Mexico City, this product is being illegally used for aesthetic treatment with fatal results. The aim of this study is to confirm the lethal toxicity documented in humans. Male Swiss Webster mice (CFW) 30-45g were employed. Dose-response curves to guaiacol were performed by subcutaneous administration (6.25-400 microl/40g). Basal temperature was recorded 30-120 min following administration of guaiacol. Animals were continuously observed for 120 min after guaiacol administration, lethality and toxicity manifestations were recorded. Depending of the dose, high toxicity was observed; sub lethal doses (6.25-12.5 microl/40 g) produced tachycardia and hyperactivity, follow by sedation, hypnosis, high hypothermic effect (loss of 6 degrees C) dyspnea, myoclonus, hematuria, blindness, abdominal distension and in higher doses (25-400 microl/40 g) lethal effect. Necropsy showed hepatic and renal necrosis, pulmonary edema, hemorrhages and bladder clotting. We concluded that guaiacol is an extremely toxic product (toxic rating class 5) whose use should be restricted or banned.