There is no causal association that I have been able to find between Ritalin (methylphenidate) and nocturnal enuresis (bedwetting). It is not a reported side effect of the medication. In fact, it has been observed in many studies that in children who have ADHD and enuresis, methylphenidate helps to improve both conditions. Other stimulants besides Ritalin have also been shown to help resolve bedwetting in children, though they are generally not prescribed solely for that purpose.

Most nocturnal enuresis is familial, meaning it runs in the family. It is felt to be due to a combination of the child being a very deep sleeper as well as having delayed maturation of the muscles that control the bladder. Children who sleep deeply just have a harder time recognizing the signals that their full bladder is sending to their brain. Most boys who have enuresis have a father, uncle, or brother who had it, and most tend to "outgrow" their bedwetting by age 13 or 14. Enuresis is about 4 times more common in boys than in girls.

There is, however, an increased incidence of enuresis in children who have ADHD. Various studies have shown that children with ADHD are two to three times more likely to have enuresis than children who don't have ADHD. Other learning disabilities and developmental disorders are also associated with a higher incidence of bedwetting.