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The pharmacokinetics of two commercial brands of intramuscular acetaminophen formulation, with andwithout lidocaine, were studied and compared in nine healthy Thai male volunteers after a single intramuscular dose. No significant differences (P >0. 05) in the volume of distribution (Vd), plasma drug clearance( Clp) and area under the concentrationtime curve (AUC) between the two formulations were observed, however, there were significant differences (P <0.05) in the absorption rate constant (Ka), abso1ption half-life [t1/2 (abs)], and peak plasma concentration (Cmax)between the two intramuscular acetaminophen formulations (3. 79±1.32 h-1, 0.20 ± 0.07 h, and 4.08 ± 0.52μg.mf1 for acetaminophen 300 mg VS 2.55 ±0.66 h-1, 0.29 ±0.07 h, and 3.68 ±0.45 μg.mt1for acetaminophen 300 mg plus lidocaine 20 mg, respectively) used in this study. These data indicate that the abso1ption rate of acetaminophen in the formulation with lidocaine was significantly reduced. The Cmaxacetaminophen in the presence of lidocaine was also reduced (P <0.05). However, the Cmax of these two formulations were lower than the lower limit of plasma acetaminophen effective concentrations (10-20 μg.ml-1). Therefore, it is doubtful whether these two intramuscular acetaminophen formulations are useful for their analgesic antipyretic actions.
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