'If buttock fat is too thick for an intramuscular injection, there may be little alternative other than selecting an alternative route of administration ... such as intravenous drug administration.' Or, the medicine may be injected in the quadriceps muscle of the thigh or other sites.

"We are the first to report that the majority of intramuscular injections into the buttocks are not effective in the Western adult population," said Victoria O. Chan, a researchers at The Adelaide and Meath Hospital in Dublin, Ireland, and the lead researcher on the study.

Radiological Society of North America in Chicago.

Intramuscular Injections

In the study, Chan and her colleagues evaluated 50 patients, half of them men and half women, ranging in age from 21 to 87. All received medicine by intramuscular buttock injection and all were scheduled for CT scans.

The researchers injected the medicine along with a milliliter of air and then did a CT scan to establish the location of the air bubble and the medicine -- helping them spot whether the medicine landed in the muscle, where it could be absorbed, or whether it remained lodged in fat.

The upper, outer quadrant of the buttock is preferred for intramuscular injections of medicine. This area has relatively few nerves, bones and major blood vessels that could be harmed by the needle, but the multitude of blood vessels in the area means drugs are easily absorbed into the system.

However, in their research, Chan found only 32 percent of the injected medications actually reached the muscles. Shots missed the mark much more often in women than in men, she added.

Clinical Judgment

Overall, 56 percent of needles given to men reached their glutes, compared to just 8 percent of those given to women. Only two of the 25 women received injected medicines that reached their muscle.

It's difficult to know how much buttocks fat is too fat for the average injection, Chan said.

"A best estimate of fat thickness can be made by depressing the upper outer quadrant of the buttock. Bearing in mind that the length of the needle used is approximately 3 centimeters (1.18 inch) it follows that the more obese the patient, the less likely the needle will reach the muscle underneath the fat," she added.

"The medical professional will have to use his or her clinical judgment with regard to fat thickness in both males and females," she said.

Those with excess buttocks fat may require a longer needle for the injection to work effectively, she said.

Another expert, Dr. William E. Palmer, director of musculoskeletal imaging at Massachusetts General Hospital, Boston, said that nurses experienced in giving injections into the buttocks "know all about this. The nurses are performing the injections and can see the effect or lack of effect."

Patients with ample posteriors who notice a medicine injected there isn't working should mention this to their doctor, Palmer said.

Alternative Route

What's the solution to this problem, besides a longer needle?

"If buttock fat is too thick for an intramuscular injection, there may be little alternative other than selecting an alternative route of administration, if that is possible, such as intravenous drug administration," Palmer said. Or, the medicine may be injected in the quadriceps muscle of the thigh or other sites, he added.

Both Chan and Palmer said it's impossible to define a body mass index (BMI) "threshold," where patients over that weight will have trouble with buttocks injections.

"Very thin patients can have thick buttock fat," Palmer pointed out, "whereas morbidly obese patients can carry their weight around the abdomen and have tiny little buttocks."

Doctors and nurses should evaluate what's best on a case-by-case basis, he said.

More information: To learn more about obesity and how to control it, head to the The Obesity Society.