Shot in the Buttocks Misses the Mark

CHICAGO (Reuters Health) - Too much fat in the buttocks may reduce the efficacy of intramuscular injections, according to research reported at the 91st annual meeting of the Radiological Society of North America.

In a study of 50 healthy volunteers buttocks injections reached muscle in just "8 percent of women and 32 percent of men in our study," said Dr. Victoria Chan, registrar in clinical medicine at the Adelaide and Meath Hospital, Dublin, Ireland.

Researchers weighed and measured 50 patients, and then injected them with tiny air bubbles that could be seen on computer-assisted tomography scans to see if the bubbles reached the muscle layer using a standard 23-gauge needle.

In most cases, the needles did not penetrate the fat layer -- even in half of the six patients who would have been considered underweight -- having a body mass index (BMI) of less than 20.

She blamed the problem on the natural hourglass shape of women and the overall obesity in the population. For example, seven volunteers had body mass indices of 35 or more and were classified as obese. In that subset "none of the injections reached muscle."

Intramuscular injections in the buttocks are commonly administered for delivery of drugs used for treatment of pain, nausea, vitamin B-12 deficiency, cancer, vaccines and contraception.

"Our study has demonstrated that a majority of people -- especially women -- are not getting the proper dosage from injections to the buttocks," said Chan in a press briefing.

"This a cautionary tale," said Dr. Michael Brant-Zawadski, medical director of radiology at Hoag Memorial Hospital Presbyterian, Newport Beach, California. "Perhaps the upper arm may be a better site than the buttocks in heavier people or in women, or perhaps longer needles are the way to get medicine where it needs to be."