Prostate cancer treatments may shorten the penile length according to a report in the Journal of Urology. 47 men were treated for varying degrees of prostate cancer and just before this the average full penile length was 5.6 inches.
A year and a half later the average penile length was 3.4 inches. Before therapy 23% of the men had normal erections, but then eighteen months later only 12.5% were able to have erectile function that was good enough for intercourse.
This is the first study of it's kind to determine if penile length changes following these sorts of treatments.Source