This systematic review from the 2020 International J of Dermatology is helpful.
There is no one protocol and the dosages range from 1.25 mg a day to 5 mg twice daily. In women, often in conjunction with 25 mg of spironolactone. There are no randomly controlled double blind studies. It would make sense to start at 1.25 to 2.5 mg a day and give it three months before changing anything. The only side-effect mentioned in the studies looked at for this afticle was some increase in facial hair,
Download Minoxidil IJD Sustematic Rev 2020
The article concludes" "Oral minoxidil is a safe and successful treatment of androgenic alopecia and Alopecia Areata. In addition to its therapeutic benefits, practical advantages over topical minoxidil stem from improved patient compliance."