Minoxidil could cause a typical allergic reaction (hives, swelling, itching, redness) or a severe one (where the typical reaction turns deadly), but so could asprin. Just be aware when you first start taking it that yes, like many other things, some people are allergic. It also might cause dandruff because the alcohol has a drying effect. If you are allergic to propylene glycol, don’t use minoxidil!
If you do have a cat, take great caution to use minoxidil in a room that cat doesn’t go in or make your bathroom cat-free. If you spill it, clean it up well. Just a few licks of minoxidil and fluffy could be dead in days. And if your cat likes to lick your head, a.) it’s kind of strange and b.) break your cat of this habit before you start monoxodil. Obviously if your cat licks your head you love your cat a lot, so really take this danger seriously.