I have a small red circumscribed patch on the glans of my penis for 3 weeks now. i had no pain, no blisters no itchiness and no generalized symptoms.

Red Patch on Penis: A red patch on the head of the penis can arise for several reasons. If you are not circumcised, this could be inflammation, called balanitis. Steroid creams help. If you have started any new meds, it could be a fixed drug eruption. Stopping the meds helps, but it is a harmless event. If it is growing in size and doesn't respond to the above in a week or two, i recommend a urological evaluation.