This randomized, double-blind, placebo-controlled study compared the time to onset of analgesia and the analgesic efficacy of two formulations of acetaminophen 1000 mg--an effervescent solution and tablet--in 242 patients with moderate or severe pain following dental surgery. Onset of analgesia was determined using a two-stopwatch procedure. Analgesia was assessed over a 4-hour period. Treatments were compared using standard indexes of pain intensity and pain relief and summary measures. Both acetaminophen formulations were significantly more effective than their corresponding placebo for all efficacy assessments. The median time to onset of analgesia was significantly shorter with effervescent acetaminophen (20 minutes) compared to tablet acetaminophen (45 minutes). During the first 45 minutes after administration, effervescent acetaminophen was significantly more effective at each scheduled assessment time than tablet acetaminophen. The median time to meaningful pain relief was significantly shorter with effervescent acetaminophen (45 minutes) compared to tablet acetaminophen (60 minutes). At 4 hours after administration, the pain relief was significantly better with tablet acetaminophen than with effervescent acetaminophen. No other significant differences were observed between the active treatments. In conclusion, effervescent acetaminophen produces a significantly faster onset of analgesia than tablet acetaminophen.