Veterinary injectable L-carnitine growing in popularity with steroid starved bodybuilders in Australia.

GYM junkies are injecting drugs meant for greyhounds in the mistaken belief it will help them build bigger muscles faster.

Although L-carnitine is found in supplements sold for human consumption, users are buying an injectable form from pet shops which experts say can increase the risk of heart attacks.

In theory, injecting the dog version of L-carnitine could help bodybuilders pump more weights before exhausting their muscles. The drug is not a steroid but is said to speed the metabolic process of converting fat into energy.

For humans it is sold as a weight-loss aid and to develop lean muscle mass.

But Wentworth Park greyhound racing's head veterinarian Ted Humphries said, besides the obvious "insanity" of injecting a drug designed for dogs, the canine version was mainly used to help muscle contraction.

He said tests of the drug on greyhounds had found "no demonstrable improvements" on performance.

"About 10 per cent of trainers use it but the other 90 per cent wouldn't bother," Mr Humphries said.

"It could cause cardiac arrest in large doses in those with an underlying heart condition. A lot of people are unaware they have one and bodybuilders would not be exempt from that."

Mr Humphries said veterinary drug manufacturers were "generally less stringent" about the exact contents of drugs compared with drugs for humans.

Personal trainer Michael Clubb said it was the latest "fad" in amateur body building and its use was emerging in Sydney's west.

"It's mostly young men. Young guys looking to bulk up and get ripped before summer," he said.

Like steroids, Mr Clubb said there were inherent and unknown risks in people injecting L-carnitine.

There are 92 medicines listed as low-risk by the Therapeutic Goods Administration containing L-carnitine and all are taken orally.

A TGA spokeswoman said injectable L-carnitine was not approved for human use and it was illegal to use veterinary drugs on people. A 100ml bottle of L-carnitine can be bought over the veterinary counter for $69.

The Australian Pesticides and Veterinary Medicines Authority said, because L-carnitine was not a schedule 4 substance, which are remedies prescribed following a veterinary diagnosis, it could not further regulate its sale. "Chemical abuse extends beyond L-carnitine and can be tragic in its consequences," a spokesman said.