Topical minoxidil can cause scalp irritation .
These were historically thought to be caused by minoxidil itself, but patch testing identified propylene glycol (present in minoxidil solution) as the causative agent in 9 of 11 patients, while only 4 of 11 reacted to minoxidil.
The substitution of butylene glycol for propylene glycol was well tolerated.
Medical treatments for male and female pattern hair loss
Journal of the American Academy of Dermatology - Volume 59, Issue 4 (October 2008)