Gizmodo - 

The most common “side effects” of masturbation are ejaculation (mostly among men, but sometimes among women); relief of boredom; fun; pleasure; relief of tension; easier time falling asleep; relaxation; and general happiness. No, you are not going to go blind.

Depending on how one masturbates, these “side effects” are also possible: monthly credit card fees, lube on one’s keyboard, drained vibrator batteries, a cucumber that should not be served to visiting houseguests, or ejaculate stains on the wall that, in the future, some investigative reporter will shine a blacklight on to demonstrate to viewers everywhere that hotels are places where people have S-E-X.

In the very rare case that you are particularly rough with your own genitals, you might be a little sore.

Be gentle with them for a few days and they should return to normal (if not, see a healthcare provider). Also in the very rare case that you insert an object into your anus and it goes too far up into the rectum, then you may have to go to the emergency room to have it safely retrieved by a doctor. This often comes with a side effect of embarrassment, but it need not — sexual pleasure and masturbation are normal parts of being human. Your X-Ray may be published (anonymously, of course) in a medical journal with a description of how the doctor creatively figured out how to get the ______ (light bulb, salad tongs, beer bottle, snake; and yes these are all from real publications) safely out of your body.

What side effects will NOT happen as a result of masturbation? You will not grow hair on your palms. You will not go blind or look excessively old. You will not gain or lose weight. There’s mixed evidence as to whether or not regular masturbation in one’s 20s and 30s decreases risk of prostate cancer later in life, but some research points in that direction.

Dr. Debby Herbenick, author of Sex Made Easy and Great In Bed, is the Co-Director of the Center for Sexual Health Promotion in the School of Public Health-Bloomington at Indiana University (IU) where she is a Research Scientist. She is also a sexual health educator at The Kinsey Institute for Research in Sex, Gender and Reproduction where she writes (and hosts audio podcasts of) the Kinsey Confidential column and coordinates educational programming. She has a PhD in Health Behavior from IU, a Master’s degree in Public Health Education (also from IU) and a bachelor’s degree in psychology from the University of Maryland, College Park. In addition, she is certified as a Sexuality Educator from the American Association of Sex Educators, Counselors and Therapists.