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Penis size is usually determined by race and genetic make up. Unfortunately, there are no medications which can increase the penis size. Size may appear different in certain conditions like phimosis and a buried penis. Uncomplicated phimosis where the foreskin is tight can be managed by medical measures, if medical measures are not helping; it warrants surgical treatment like circumcision.
A buried penis is of normal size but appears buried due to insufficient skin between the abdomen, scrotum and shaft of the penis.
In certain conditions, the size of the penis may be smaller than the normal and is usually associated with other abnormalities and it is called microphallus.
An anatomically normal but small sized penis is usually due to genetic make up and does not have any medical treatment to increase the size. Surgical treatments to lengthen the penis are available but they are of uncertain value and associated with their own complications.
If your erection is not strong enough, a physical examination and an assay of hormonal levels will help in identifying the problem. Erectile dysfunction has psychological causes. Please consult your doctor for the examination and necessary investigations, if required. If the cause of ED is a suspected psychological reason, then you may benefit from counseling. Thank you.