Injecting drugs into the buttocks may not be a reliable way of administering medicine, research suggests.
Jabs need to reach to the muscle
Doctors from a hospital in Dublin found many patients had so much fleshy tissue on their buttocks that jabs could not properly penetrate to the muscle.
They found women, and in particular obese women, were most likely not to get the full intended dose.
Details were presented to a conference of the Radiological Society of North America.
Lead researcher Dr Victoria Chan, based the Adelaide and Meath Hospital, said: "Our study has demonstrated that a majority of people, especially women, are not getting the proper dosage from injections to the buttocks.
"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."
Many medications are administered through injections into the muscles of the buttocks, including painkillers, vaccines, contraceptives and anti-nausea drugs.
The buttock is the preferred site because it contains relatively few major blood vessels, nerves or bones that could be damaged by the needle, but the underlying muscle has a rich supply of microscopic blood vessels which can absorb medicines effectively.
Used to administer:
Some forms of contraception
Long-acting cancer drugs
Intramuscular injections are a common alternative when patients cannot take pills.
The use of injections has increased over the past 10 years and new medications have been developed for delivery in this way.
However, Dr Chan's research found 68% of the injections do not reach the muscles of the buttock.
She said: "The amount of fat tissue overlying the muscles exceeds the length of the needles commonly used for these injections."
Pharmaceutical companies design medications based on the assumption that they are injected straight into the muscle.
Injections which only reach into the fat tissue will not deliver as much medication, as the tissue contains significantly fewer blood vessels.
Dr Chan said her work suggested that patients were either not receiving the maximum benefit of a drug or receiving no benefit at all.
Furthermore, if the medication is not absorbed into the blood stream, it remains in the fatty tissue where it can cause local infection and irritation.
The research focused on 50 patients due to undergo abdomen or pelvis scans.
Each was given an intramuscular jab which contained a small air bubble so the researchers could track the path of the medication when they carried out the scans.
Only 56% of injections successfully reached muscle tissue in men - while in women the success rate was just 8%.
Compared to men, women typically have a higher amount of fat in their buttocks.
Dr Chan said longer needles would be need to increase the success rate of the jabs.
Professor Richard Guy, an expert in pharmaceutical sciences at Bath University, told the BBC News website the effectiveness of vaccines could be particularly compromised.
They tend to be made up of large molecules that would only slowly diffuse out of fat tissue.
He said: "Whether using longer needles is a practical solution, I'm not sure, as these are unlikely to be terribly popular.
"A better approach might be to give the intramuscular injection where there is less fat around."
Professor Guy said alternative technologies, such as micro needles, were under development which can deliver drugs into the blood vessels underlying the skin.
Dr Jim Kennedy, prescribing spokesman for the Royal College of General Practitioners, said intramuscular injections were not in as common use in the UK as other parts of Europe.
He said obesity was doubtless a factor but it was also important to use the right length of needle and to ensure that those who administered jabs had proper technique.