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Phentermine: What is the substance Michelle Payne was taking, and why is it banned?

ABC News logo ABC News 29/06/2017
Michelle Payne had been taking the substance on and off for two months. © AAP Image: Darren McNamara Michelle Payne had been taking the substance on and off for two months.

Jockey Michelle Payne won't be allowed to race for a month after testing positive for Phentermine, a banned substance.

The Melbourne Cup winner's urine sample from the Swan Hill Cup meeting on June 11 returned a positive result for the appetite suppressant, according to Racing Victoria.

She has taken "full responsibility" for the oversight, and said she regretted not making more inquiries about the drug, which prompts the question:

What is Phentermine?

The substance, manufactured by a number of pharmaceutical companies in Australia, is banned under the Australian Rules of Racing.

It is used in Australia and worldwide as a short-term treatment for obesity, and works by stimulating the release of chemicals, such as norepinephrine, in the body.

"It gives you the sense of being full, so it reduces the perception of hunger," Timothy Fairchild, a senior lecturer in exercise physiology at Murdoch University, said.

The drug was prescribed to Payne in the wake of a bad fall in May last year. She needed pancreatic surgery after the horse she was riding, Dutch Courage, stepped on her during a race in Mildura.

"It is clinically useful in obese individuals, and for that reason it is prescribed for that person," Dr Fairchild said.

"Whether it would lead to [weight] benefits in individuals who are not obese is probably questionable."

Why is it banned?

"Any appetite suppressant is banned in the wider codes," a spokeswoman for Exercise and Sports Science Australia (ESSA) told the ABC.

"It's seen to be a way of getting ahead, particularly in sports where there is a weight category." She said the drug could cause issues for boxers and gymnasts for the same reason.

"On the flipside, it is also about the health and wellbeing of the athlete."

A Racing Victoria inquiry on Thursday was told Payne had been taking the substance on and off for two months.

According to the US National Institutes of Health (NIH), most people take Phentermine for three to six weeks.

The International Programme on Chemical Safety says the drug has a "high risk of dependency and abuse".

It can also have serious side effects, including increased blood pressure and heart palpitations, the NIH says on its website.

"You'll get some heart tremors, restlessness, [or] sleeplessness, because it may also stimulate your metabolic rate," Dr Fairchild said. "So, people do need to be careful."

He said there were complex differences in what was banned and not banned across sporting codes, and pointed to archery and shooting as an example.

"In archery and shooting, where precision is required, beta blockers are banned — even though it is a common medication and saves people's lives — because it brings heart rate down [and] lengthens the time between beats, and therefore anyone in these precision sports has a longer time in which to shoot between heartbeats," he said.

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