Shots often don't reach muscle.

Standard 3-centimeter needles are too short to penetrate the layer of fat in the buttocks of most women and most obese men, body scans reveal. Medications that physicians think they're administering to muscle, therefore, often don't reach that target.

A recent study by researchers in Ireland found a failure rate of 92 percent in women and 44 percent in men.

"Overall, 68 percent of the injections failed to reach the muscle," says Victoria Chan of the Adelaide and Meath Hospital in Dublin. "We recommend first and foremost the use of a longer needle, especially in women" Women typically have more buttocks fat than men do.

The researchers studied 25 patients of each sex who, for medical reasons unrelated to the study, received injections in the rear and then underwent computed tomography of the abdomen or pelvis. Because an injection creates tiny bubbles at a needle's tip, the scans revealed how far the needles had penetrated.--B.H.