40 days won't mean much, I don't think a less itchy scalp could mean anything that early either really (but everyone's different, who knows).
Sorry to get so personal, but if you aren't regularly having sex then don't worry about decreased sex drive, go on 1 mg, if you are definitely feeling decreased ability after a few months, try less, and if it persists, go off it and find another way. I somewhat doubt it can change your hormones in only 6 weeks to have decreased sex drive, but yeah, everyone's different, and the only way you'll really know is to take the plunge.