Tolfenamic acid (TA), a potent inhibitor of prostaglandin (PG) biosynthesis and action, was tested prophylactically against hangover symptoms in 30 healthy volunteers in a double-blind cross-over study. One capsule of TA (200 mg) or placebo was taken before starting to drink alcohol and another before going to bed. The hangover symptoms were evaluated in the morning. TA was found significantly better than placebo in the subjective evaluation of drug efficacy (p less than 0.001) and in reducing the reported hangover symptoms in general (p less than 0.01). In the TA group, significantly lower symptom scores were obtained for headache (p less than 0.01), and for nausea, vomiting, irritation, tremor, thirst and dryness of mouth (all p less than 0.05). In a separate study with eight participants, plasma levels of PGs were followed during ingestion of alcohol with or without TA. The plasma concentrations of PGE2 and TXB2 (a metabolite of thromboxane A2) were lower in the TA group during alcohol ingestion, while PGF2 alpha and 6-keto-PGF1 alpha (a metabolite of prostacyclin) were unaffected. TXB2 correlated with blood alcohol levels in a U-shaped manner.