Common causes of blisters on the side of the penis include genital herpes and warts. 1. Genital herpes: Genital herpes is a sexually transmitted disease caused by herpes simplex virus infection, which is easily recurring and difficult to cure. The main symptoms include blisters and ulcers in the genital area, accompanied by itching and pain. 2. Condyloma acuminatum: Condyloma acuminatum is a sexually transmitted disease caused by human papillomavirus infection and the formation of wart-like infections. Typical symptoms include single or multiple small particles, blisters, etc. on the male glans, urethra, and prepuce, in the form of cauliflower, corns, and so on. In addition to this, there are many other causes that can cause similar symptoms, and when patients experience discomfort, they should go to the hospital immediately.