A diabetes drug in short supply for almost a year has led to what one doctor has described as "The Hunger Games" for patients trying to fill their prescriptions.

Semaglutide, a weekly injection sold under the name Ozempic, was approved by the Therapeutic Goods Administration (TGA) for lowering blood sugar in adults with type 2 diabetes, but can be prescribed off-label to treat other conditions.

There has been a global shortage of the drug for almost a year, and the TGA advised last November that the drug would be unavailable until an expected return to regular supply at the end of March.

The TGA has linked the global shortage with an unexpected increase in demand due to off-label prescribing for weight loss.

South Australian GP Danny Byrne said doctors had been advised to limit prescriptions to people with type 2 diabetes during the shortage.

And while the TGA can make such recommendations, it does not have the power to enforce them.

"They've no legal power, it's a high recommendation," Dr Byrne told ABC Radio Adelaide's Stacey Lee and Nikolai Beilharz.

"So, my personal ethics are that I won't prescribe it for weight loss, but there's no law that stops me from doing that."

Dr Byrne said off-label use of the drug was not covered by the government-funded Pharmaceutical Benefit Scheme (PBS) and was therefore much more expensive — about $130 per month compared to a maximum of $30 for type 2 diabetes patients.

He said that price difference meant pharmacists could tell whether it was a script for type 2 diabetes treatment, or for an off-label use.

Dr Byrne said he would consider prescribing the drug for weight loss when normal supply resumed — but only in very specific cases.

"We've got this term we call 'morbid obesity', that's where your weight is actually going to impact on your health and lifestyle and death risk as well," he said.

"So, I've got patients that are 140, 160 kilos and they will be first in line for Ozempic once it becomes more available in about April."

Dr Byrne said there were alternative options for other people who wanted to lose weight.

"Why would you go on an injection every month for the rest of your life just to lose five kilos when it's got side effects, it's very expensive, $130 a month forever, and as soon as you stop using it, the weight's going to come back on plus more often as well," he said.

Luke was prescribed Ozempic off-label to use prior to surgery.

"I've got a long-term thyroid lump in my neck and I'm overweight, so before Ozempic became fashionable I was taking it," he told ABC Radio Adelaide.

But like many others, Luke has struggled to access the drug since the shortage.

He went back to the doctor recently and despite receiving a script, he found it was unavailable.

"The long-term gain is so I can get this lump cut out of my neck without further complications of being overweight," he said.

The ABC has contacted the Pharmacy Guild of SA for comment.