As if baring one's backside to get an injection weren't bad enough, a new study, described at this week's annual meeting of the Radiological Society of North America, adds medical insult to ego injury: Such injections often don't work.

People have become increasingly obese, and the study, done at the Adelaide and Meath Hospital in Dublin, Ireland, found that the standard 1¼-inch needle is too short to reach through the fat into the muscle tissue beneath to deliver its payload. Or, as Victoria Chan delicately put it in describing the key finding of the study she and her colleagues ran, "needle length hasn't adapted to larger body sizes."

Painkillers, antinausea drugs, and other medications typically are administered through a needle into buttocks muscle. Major blood vessels and nerves are few, so the risk of damage is low. And muscle tissue, which has a rich blood supply that can circulate the medication rapidly, is abundant in that part of the body.

Radiologists at the hospital became curious, however, when they noticed that in many patients getting CT scans, a small air bubble, which normally appears at the end of the needle when a drug is injected, was showing up in fat and not muscle. Over a six-month period, the study team enrolled 25 male and 25 female patients scheduled to receive an abdominal or pelvic CT scan along with an injection in the buttocks.

The bottom line: The drug reached its destination in only 16 of the 50 patients, and in none of the seven patients with a body mass index above 30, which is considered obese. Moreover, most of the successful injections were in men; the shots worked in just two of the 25 women. It wasn't that most of the women but few of the men were obese; the reason, said Chan, is "women's hourglass figures." That is, women have more fat in their backsides.

One solution, said Chan, would be to switch to the upper arm or thigh. Another would be development of drugs that are better absorbed in fatty tissue. And, of course, longer needles could be employed—without, Chan assured, any increase in risk.