Oh god, for all the people suggesting Minoxidil:
Minoxidil is used to treat a very specific kind of hair loss caused by damaged hair follicles due to overproduction of testosterone (androgenic alopecia for the nerds out there). This usually occurs due to genetics, aging, or stress where the excess testosterone weakens then kills the hair follicles. When used early (within 1-2 months of initial hair loss), treatment is about 40% effective for HAIR LOSS because there are so many other different mechanisms for losing hair in the body. In short, it can't revive dead follicles, replace them with new ones, or magically cause more to appear.
The idea that minoxidil would help hair growth comes from the idea that because minoxidil is also a vasodilator, it allows "extra" blood and oxygen to reach the hair follicle to help it produce more keratin. The clinical evidence for this is mixed at best, and you can bet big pharma would be advertising this like the cash cow it is if it worked (think about why there are no ads for penis enlargement pills).
I wouldn't normally say anything without presenting an alternative, but minoxidil is rather expensive (a 2-month treatment is like $50 if you're using it correctly, and you need to use it for ~6 months to start seeing results) compared to other folklore-inspired treatments (ex: essential oils). For the regular hair loss case, I usually tell people wigs are cheaper, last longer, and produce instant results.
Source: interned at a pharma company; this is based on their internal clinical results.