A new injectable treatment is promising to get rid of the dreaded double chin - for good.

Injectable, synthetic deoxycholic acid has just been approved by MedSafe NZ for the removal of "submental fullness" - the pad of fat underneath the chin. The acid is produced by pharmaceutical company Allergan and marketed under the name Belkyra.

It is actually a synthetic version of an acid found in human bile, said Dr Sarah Hart, a cosmetic physician at the Skin Institute in Auckland.

"When you eat a fatty meal, bile is released from the gall bladder, including deoxycholic acid. Its job is to digest fat," Hart said.

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"If you inject it under the chin, it will preferentially burst fat cells like little balloons because it's attracted to that fat."

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The molecule was first trialled by dermatologists in Australia a decade ago, before the tech was released in the United States in 2015 as Kybella. It is now also available in Canada, Australia and parts of Europe.

While some gain a second chin when they age, and others when they put on weight, a third group of people are genetically programmed to get a double chin even if they stay very slim, Hart said.

The number of fat cells in the body is set early in life - usually around puberty, Hart said.

"After that you don't grow new ones. If you put on weight, each cell gets bigger. This treatment will reduce the number of fat cells in that area - and once they're gone, they're gone."

The treatment takes about 20 minutes, and for most people it involves 20-40 injections. Patients' chins tend to swell up for weeks afterward as the lymphatic system removes the destroyed fat cells, which are excreted. A second treatment is due after about eight weeks.

There is not likely to be further treatment needed, although a top-up injection after four years can be helpful.

MedSafe lists more serious, though rare, side effects. They include possible allergic reaction, trouble swallowing, an uneven smile, discolouration, ulcers or taste disturbance.

The treatment must be prescribed by a doctor and it will not be publicly funded. Costs are not yet known but in Australia it costs about $2600 for the first two treatments.

Hart said the treatment was not about vanity.

"People feel their appearance impacts on the way they're treated. As they age, they look tired, sad or angry when they're not. They have frown lines, their mouth droops, they look miserable - especially in their careers, that impacts on the impression they give."

So far deoxycholic acid has only been approved for submental fullness but researchers are looking at other possibilities, Hart said.

"It's a pretty quickly developing area of medicine - and it's powerful. I used to work in mental health and I've been incredibly pleased and surprised at how much making someone look better makes them feel better.

"When they feel better they take care of their health. It's certainly better than shaming people."