Johnson & Johnson's (NYSE:JNJ) attention deficit hyperactivity disorder (ADHD) pill, Concerta, is growing up. On Friday, the Food and Drug Administration approved the drug for adults.

Concerta, a long-acting version of Novartis' (NYSE:NVS) Ritalin, joins Eli Lilly's (NYSE:LLY) Strattera and Shire's (NASDAQ:SHPGY) Vyvanse -- also  approved to treat adults.

Contrary to what you might think, ADHD isn't just for kids anymore. The potential market size for adult attention deficit disorder (ADD) is 30% to 70% of the childhood market. As many as one in 20 adults has ADD.

The biggest problem for drug manufacturers is getting those potential patients onto their drugs. With a stigma that rivals that of erectile dysfunction, manufacturers will be hard-pressed to get prospective patients to talk to their doctors about this problem. Even the company-sponsored informational websites, support groups, and -- I kid you not -- a mobile awareness tour probably won't be enough to bring in all of the potential patients.

Still, as Shire has shown, there's plenty of money to be made from treating ADHD kids and adults. The company's three ADHD drugs -- Adderall XR, Vyvanse, and Daytrana -- combined for more than $336 million in sales last quarter, on their way to a combined blockbuster status this year. Johnson & Johnson was just behind Shire with $290 million in sales of Concerta last quarter.

It's anyone's guess how much being able to market the drug to doctors for use in adults is really going to help sales of Concerta, especially because there's no way to know how much of its current sales is for off-label use in adults. Investors should keep an eye on sales of Concerta to see if being able to advertise to this more mature market really helps Johnson & Johnson.