Baldness is usually from a few things.
1. hormonal imbalances (usually in testosterone)
2. liver/portal congestion
3. chronic inflammation (from pathogens, food sensitivities, bad diet, etc)
4. Nutrient deficiencies (vitamins, minerals and proteins)
There is no 1 or 2 supplements that one can take to reverse it or stop it as several protocols need to be done to address the root cause. DHT blockers such as saw palmetto can be taken but usually you want to do drainage, detoxification and take a good multivitamin and multimineral supplement.