Genetics play a huge role on side effects during a steroid cycle, including how much hair you will lose. Some will experience a little increase in shedding during a heavy androgenic cycle, while others will practically see clumps come out when they comb.
Bodybuilders usually report though that it starts to thicken back weeks after the cycle is over with. The thing about male pattern baldness is that androgenic alopecia is a condition that gets worse. Just because it grows back doesn’t mean it is the same as before. That is because DHT (or any androgenic steroid) causes follicular miniaturization on hair follicles. Meaning, the hairs get progressively thinner and thinner everytime, until eventually its too thin to see or stops growing completely.
Since DHT and steroids can do irreversible damage to the hair follicles via this follicular miniarturization mechanism, the best way to prevent the damage is blocking via anti-androgens on the scalp. Two common shampoos are Spironolactone and Nizoral, that act as anti-androgens in the scalp. Nizoral 1% can be found in stores, the stronger 2% is via prescription, although over the counter in Canada. Spironolactone is found commercially as a pill taken orally, but obviously it’s anti androgen properties would counteract your steroids, so there are sites that sell it in topical solution for your hair.
The prescriptions Finasteride (Brand names Propecia, Proscar, etc) block the conversion of testosterone to DHT via the 5AR enzyme. This will Not be helpful at all for steroid cycles consisting of Tren, D-bol, Anadrol, or any other steroid besides testosterone that cause hair loss on their own. Taking Finasteride with Deca would even worsen your hair loss, since Deca converts to a very weak androgen DHN in the scalp and prostate, weaker androgenically than deca itself and these 5AR blockers would stop that conversion.
The popular over the counter and prescription treatment Rogaine won’t do much to prevent it while on cycle. It is more for getting your hair back after the fact. Anti-dandruff shampoos with zinc pyrithione, have shown in studies to have a hair diameter increasing effect, so they should be used regurlarly on and off your steroid cycle for good measure. There is also some hair growth supplements available on the market.