Cimetidine increases the plasma concentration of hydroxyzine.

The combination of hydroxyzine and cimetidine is reported to be more effective than hydroxyzine alone in chronic urticaria. The plasma concentration of hydroxyzine was studied in seven patients treated both with the hydroxyzine 25 mg t.i.d. and the combinations of hydroxyzine 25 mg and cimetidine 200 mg t.i.d. Hydroxyzine plasma concentrations were definitely higher during the combination treatment than during hydroxyzine alone. This may explain the effectiveness of the combination treatment.