Objective: The aim of this study was to compare the dose-related effects of both ibuprofen tablets and ibuprofen effervescent [placebo, 400 and 800 mg ibuprofen (Aktren)] on phasic pain.Patients: Twenty volunteers participated in this randomized, double-dummy, fivefold crossover study.Methods: Measurements were obtained before and 15, 60 and 240 min after drug administration. Pain was produced by CO2 pulses applied to the left nostril. Subjects rated the intensity of the painful stimuli by means of a visual analogue scale. In addition, chemosomatosensory event-related potentials were recorded.Results: In line with previous work, ibuprofen produced a dose-related decrease in pain-related potential amplitudes P1N1, indicating its antinociceptive effects. Higher plasma concentrations of ibuprofen were reached 15-40 min after administration of the effervescent while ibuprofen tablets had a t(max) 60-90 min after administration. In addition, 60 min after intake of the effervescent a prolongation of the latencies of the potentials was observed, possibly reflecting superior antinociceptive properties when compared to ibuprofen tablets. In addition, the effervescent appeared to have more consistent effects on intensity estimates compared to ibuprofen tablets.