While oral minoxidil is considered an “off-label” treatment for male pattern baldness, it is FDA-approved (at higher doses) to treat high blood pressure.
Doctors began prescribing low-dose oral minoxidil tablets as a convenient, once-a-day hair loss treatment because in clinical trials studying higher doses of oral minoxidil for high blood pressure, 80% of patients experienced hypertrichosis—"elongation, thickening, and enhanced pigmentation of fine body hair,” including on the scalp.
Healthcare providers have the discretion to prescribe medications off-label if they believe it is an appropriate course of treatment for a particular patient.