Currently there is a shortage of Ozempic due to high demand and some doctors prescribing off-label, as it is not licensed for weight loss.
While Ozempic, available on the NHS, helps people with type 2 diabetes, its ability to suppress appetite has led to people using it to lose weight.
This has led to a shortage of the drug as well as an increase in counterfeit jabs.
Wegovy, another brand of semaglutide, is licensed for weight management and is also available on the NHS.
The NPA, which represents 5,000 independent community pharmacies across the UK, is urging people to speak to a pharmacist or GP instead of buying medicines online from sellers who are not registered or regulated in the UK.
It also warned the shortage of Ozempic, made by pharmaceutical company Novo Nordisk, is likely to continue into next year.
NPA chairman Nick Kaye said: “Pharmacists remain deeply concerned that the current medicine shortages crisis could lead to an explosion in the unlicensed sale of medication online.
“Stocks of Ozempic are very depleted in community pharmacy in the UK and it is important that these remain prioritised for those in the most clinical need.
“Given the precarious state of supply of this and other vital medication, there is a much greater risk of people looking to order in supplies from disreputable online vendors.”
Last year the Medicines and Healthcare products Regulatory Agency (MHRA) reported a very small number of people being hospitalised, including for hypoglycaemic shock and coma, after using potentially fake pens.
In January, the Department of Health and Social Care told healthcare providers not to prescribe the drugs off-label for weight loss and said existing stock must be reserved for patients with type 2 diabetes.
It said the supply issues were being caused by increased demand and off-label use.
“We don’t know how prevalent off-label prescribing is but we do know [it happens],” Mr Kaye said.
He suggested people can check to see if an online provider is registered by the General Pharmaceutical Council or Pharmaceutical Society of Northern Ireland.
A Department of Health and Social Care spokesman said: “We strongly advise the public not to buy regulated medicines from unauthorised online retailers or beauty salons as they could be dangerous.”
It added the MHRA would continue to take action against those unlawfully trading in medicines, and said that the government was trying to tackle the obesity crisis by shifting the focus from treatment to prevention.
