Methaqualone-diphenhydramine interaction study in humans.

Twelve healthy subjects received three single oral doses (250 mg) of methaqualone alone or in combination with diphenhydramine (25 mg). Blood samples were collected for a 48-hr period after each dose and analyzed for methaqualone and its major metabolite, 2-methyl-3-(2'-hydroxymethylphenyl)-4(3H)-quinazolinone. Peak blood concentrations ranging from 1.0 to 2.7 micrograms/ml occurred approximately 1-2 hr after the oral dose. The area under the blood level-time curve, peak plasma level, and elimination half-life for methaqualone were not significantly different (three-way ANOVA, p greater than 0.05) when methaqualone was administered alone, in combination with a diphenhydramine elixir or as a commercial product (capsule) containing both methaqualone and diphenhydramine. Statistically significant intersubject differences in the area under the curve were eliminated if the area was corrected for subject differences in elimination. Blood levels of the metabolite reached an average peak of 314 ng/ml (+/- 107) between 4 and 8 hr after the dose and remained elevated for the 48-hr sampling period. The areas under the blood level time curve of the metabolite were not significantly different for the three treatments. Diphenhydramine administered at the dosage level used in therapeutic combination products did not alter the blood levels of methaqualone or its metabolite. In addition, no significant differences in methaqualone availability from the two commercial formulations tested could be detected.