PEOPLE’S bottoms are getting so big they no longer make a suitable target for injections, a study in Ireland has shown.

Many are now so obese that the needles customarily used simply are not long enough to penetrate the fat and reach the muscle, where they are aimed, Victoria Chan told the annual meeting of the Radiological Society of America in Chicago yesterday.

“Our study has demonstrated that the majority of people, especially women, are not getting the proper dosage from injections to the buttocks,” said Dr Chan, of the Adelaide and Meath Hospital in Dublin. “There is no question that obesity is the underlying cause. We have identified a new problem related, in part, to the increasing amount of fat in patients’ buttocks.”

The upper, outer quadrant of the buttocks is often chosen as a site of injection because there are relatively few large blood vessels, nerves and bones there that could be damaged by the needle. Yet the rich supply of microscopic blood vessels in muscle speeds drug absorption into the system.

Intramuscular injections are a common alternative when patients cannot swallow oral medications, are fasting for a procedure or have a metabolic disorder that inhibits the absorption of drugs taken by mouth. They have increased in recent years and medicines as varied as painkillers, vaccines, contraceptives, anti-cancer and anti-nausea drugs are all injected into the muscle.

But Dr Chan’s study shows that 68 per cent of “intramuscular” injections in the buttocks do not reach the muscle. “The amount of fat tissue overlying the muscles exceeds the length of the needles commonly used for these injections,” she said.

They discovered this by injecting 50 patients and including a small air bubble in the injection — perfectly safe as the target site was not a blood vessel. The air bubble was located by CT scans, and the team also measured body mass index, distance to injection site and thickness of fat and muscle.

Overall, the success rate of the injections was only 32 per cent. Men, who are naturally leaner in this part of the body, did better than women. More than half (56 per cent) of the injections in men reached the muscle, but in women it was a mere 8 per cent.

Drug companies design medicines so that the proper dosage is absorbed into the bloodstream from the muscle. Because fat tissue has significantly fewer blood vessels, less of the drug is absorbed or sent to the right part of the body.

“This results in the patient either not receiving the maximum benefit of the drug or receiving no benefit at all, because the drug levels are insufficient to have any effect,” Dr Chan said. There is also a danger that the drug will linger in the fat, possibly causing local irritation or infection. The answer is longer needles, Dr Chan said. The standard needle used in injections is five eighths of an inch long (16mm). Studies have shown that a 1in needle is adequate for men of all weights and women weighing between 9st 6lb (60kg) and 14st 2lb (90kg). Women heavier than that need a 1½in needle.

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