When Rogaine first hit the market back in 1988 the FDA only approved a 2% by volume mixture. Part of that was because of the paucity of long term clinical findings. After 20 years or so of widespread use with no significant contraindications coming to light higher concentrations were allowed and today 5% is widely considered to be the optimal concentration. Even so, 5% is not typically recommended for women due to its tendency to produce unwanted facial hair. (Although there are plenty of women who would rather deal with some unwanted hair on their face than not enough hair on their head.) Pregnant and nursing women are also usually advised against using the stronger concentration due to potential side effects. Essentially the stronger the concentration of Minoxidil the greater the chance of side effects.\r