Absorption of minoxidil after topical application: effect of frequency and site of application

The effect of application site and frequency on the systemic absorption of topical minoxidil was studied in 52 normal men. Subjects received 1 ml 3% minoxidil solution applied four, six, or eight times daily to the scalp or two, four, six, or eight times daily to the chest for 14 days. Serum and urine were collected and analyzed for minoxidil. No systemic minoxidil accumulation occurred from increasing application frequency to the scalp. Trends in the chest data suggest that absorption may have been lower with the twice-daily regimen. Absorption through the scalp and chest were similar for the lower-frequency regimens; however, trends in the eight-times-a-day regimens suggest that absorption may have been somewhat greater from application to the scalp. Systemic minoxidil accumulation resulting from frequent application is unlikely. The initial dose probably saturates the skin for a period of time longer than the dosing intervals examined.