Half a dose is perfectly fine and actually just as effective pretty much. You'll most likely see great with results. Just stick with it and check back after 6months to a year. Up the dosage later if need be.I've been on finasteride since my latest update. Started with 1mg/day, but I changed it to 0.5mg/day now.
I might be placebo, but I just felt better with half a dose then the full.