Adults using prescription testosterone gel should be careful not to spread it on children inadvertently, because it causes serious side-effects, the U.S. Food and Drug Administration warned Thursday.

Boys and girls who come into contact with the gel may experience:

• Inappropriate enlargement of the penis or clitoris.
• Premature development of pubic hair.
• Early aging of bones.
• Increased libido.
• Aggressive behaviour.

Testosterone gel is used by men whose bodies no longer make the sex hormone, or who have very low levels of it.

"These drugs are approved for an important medical need, but can have serious, unintended side-effects if not used properly," Dr. Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research, said in a statement.

"We must ensure that the adults using them are well-informed about the precautions needed to protect children from secondary exposure."

The FDA called for stronger warnings on the labels of Solvay's AndroGel and Auxilium Pharmaceuticals Inc.'s Testim. Both products are approved for use in Canada.

Labels on the product warn patients to wash their hands carefully after using the product and covering the treated skin — typically the upper arm or shoulder — with clothing.

As of December 1, 2008, the FDA said it had received reports of eight cases of secondary exposure to testosterone in children ranging in age from nine months to five years, primarily from direct contact between the treated skin and the child.

In some cases, children had to undergo invasive diagnostic tests. In at least one case, a child was hospitalized and had surgery due to a delay in recognizing the cause of the symptoms, the FDA said.

In most cases, signs and symptoms went away once the gel was identified as the cause and adults took precautions. In some children, however, enlarged genitals or modestly greater bone age remained abnormal.