Effect of over-the-counter dosages of naproxen sodium and acetaminophen on plasma lithium concentrations in normal volunteers.

Prescription doses of nonsteroidal antiinflammatory agents have been shown to decrease clearance and increase plasma concentrations of lithium. This study was designed to evaluate whether over-the-counter (OTC) doses of naproxen sodium or acetaminophen have the same potential to affect lithium concentration. This was a prospective, crossover, 3-phase study conducted at the Clinical Pharmacology Studies Unit of the Albany Medical Center Hospital during July and August of 1995. The 3-phase study comprised the following: phase 1, lithium carbonate (300 mg every 12 hours) alone for 7 days; phase 2, lithium and either naproxen sodium (220 mg every 8 hours) or acetaminophen (650 mg every 6 hours) for 5 days; and phase 3, a 2-day washout period followed by a crossover to lithium with the alternate drug (acetaminophen or naproxen sodium) for 5 days. Twelve healthy male volunteers were recruited, nine of whom completed the study and were included in the statistical analysis. Mean (+/-SD) plasma lithium concentrations for subjects in treatment group 1 (lithium in phase 1, lithium and naproxen sodium in phase 2, lithium and acetaminophen in phase 3) were 0.38 (+/-0.11), 0.40 (+/-0.07), and 0.36 (+/-0.11) mEq/L, respectively. Mean plasma lithium concentrations for subjects in treatment group 2 (lithium in phase 1, lithium and acetaminophen in phase 2, lithium and naproxen sodium in phase 3) were 0.43 (+/-0.05), 0.48 (+/-0.10), and 0.48 (+/-0.05) mEq/L, respectively. One-way repeated-measures analysis of variance and paired t-test showed no statistically significant differences (p>0.05) in plasma lithium concentrations during any phase of the study. The results of this study demonstrated that OTC doses of naproxen sodium and acetaminophen did not increase plasma lithium concentrations in these volunteers when taken for short periods of time.