Washington ? A little testosterone might be good for adults, but it can cause serious harm to children, federal health officials warned Thursday.

The Food and Drug Administration said adults using prescription testosterone gel must be extra careful not to get any of it on children to avoid causing serious side effects. These include enlargement of the genital organs, aggressive behavior, early aging of the bones, premature growth of pubic hair and increased sexual drive.

Boys and girls are both at risk.

The problems arise if adults don’t wash their hands well.

Also, since testosterone gel is usually applied to the upper arms or shoulders, adults must cover up to keep kids from accidentally touching a spot that has the medicine on it.

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

Health officials are recommending that adults who use testosterone gel wash their hands with warm soap and water after each use and cover their skin after the gel has dried. Pregnant women, and those who may become pregnant, should avoid any exposure, since it could lead to birth defects.