Those who take attention deficit medication could end up with members that stand to attention for painfully long periods of time.

The United States Food and Drug Administration has issued a warning that methylphenidate products, one type of stimulant drug used to treat attention deficit hyperactivity disorder may in certain circumstances cause prolonged, and sometimes painful, erections.

Known as priapism, the condition can occur in males of any age; it happens when blood in the penis becomes trapped, leading to an abnormally long-lasting erection, according to an FDA statement.

Younger males, especially those who have not yet reached puberty, quite likely won't understand the problem and feel embarrassed to tell anyone if it happens to them.

If not treated right away, however, priapism can lead to permanent damage to the penis. Therefore, the FDA recommends all males taking ADHD drugs, and their caregivers, should learn the signs and symptoms of priapism, along with the importance of seeking immediate medical treatment if it occurs.

Patients who take methylphenidate and develop erections lasting longer than four hours should seek immediate medical treatment, the FDA said.

One of the most common childhood brain disorders, ADHD can continue through adolescence and adulthood and causes symptoms such as difficulty staying focused and paying attention, difficulty controlling behavior, and hyperactivity, FDA data says.

But when medications like methylphenidate are used to treat ADHD, patients report an ability to increase focus, reduce impulsiveness and improve overall social functioning.

In an FDA review, the median age of all patients -- ranging from 8 to 33 years of age -- who were administered a methylphenidate product and then experienced priapism was 12.5 years.

In a few patients, the erection problems occurred after an increase in the dosage of methylphenidate, although the condition has also been known to happen during short periods of time when the drug was stopped temporarily, when there was a longer than typical time between doses or after stopping the drug permanently.

Priapism appeared in FDA research to be more common in patients taking atomoxetine, also used to treat ADHD, than in patients taking methylphenidate products.

Reports were filed with the FDA about four patients who took an amphetamine product and later experienced erection problems. Agency officials, however, could not be certain using the the amphetamine products caused the priapism since all of the patients in question had been taking other medications also linked to priapism.