I’m shedding 400 hairs/day after starting finasteride

I am a diffuse thinner and I started fin (1mg/day) 3 weeks ago and I am shedding hair like crazy. I would say about 300-400 hairs each day over the past week or so. Is shedding this much normal when starting fin?

If you are really shedding 400 hairs/day, this is unusually high. It might be a finasteride shed as you just started it, but it will last up to a few weeks. With that type of shed from finasteride, I would expect great results from the drug. You may not be accurate in your hair count, but it should stop shortly.