People paying privately for weight-loss drug Mounjaro in the UK face a rise in the cost of the treatment after manufacturer Eli Lilly said it was increasing the list price of the drug by as much as 170%.

It means the suggested price for a month’s supply of the highest dose of the drug will rise from £122 to £330, although the rise for lower doses will be smaller.

More generally, retailers who sell the drug may be able to negotiate discounts to the list price, so it is possible the impact on consumers will be limited.

The announcement does not affect the price the NHS pays, as the service has negotiated a heavily-discounted rate for those getting the drug on prescription.

The weekly injection works by making you feel full so you eat less, and can help people lose 20% of their body weight.

Currently there are thought to be around 1.5 million people on weight loss drugs in the UK with more than half of them on Mounjaro. Estimates suggest nine in 10 pay for these drugs privately, buying from online services and high street pharmacies.

Eli Lilly said the drug had been sold in the UK at a price that was “significantly below” that charged in other European countries.

It comes after US President Donald Trump complained about the high cost of drugs in the US.

He has threatened the pharmaceutical industry with a “most favoured nation” drug pricing proposal to peg US prices to those abroad.

Eli Lilly said it agreed with the objective expressed by President Trump that costs should be shared more fairly – although a “most favoured nation” approach was not the right answer.

Research suggests prices in the US can be three times higher than in other rich nations.

In a statement, Eli Lilly said: “The UK was one of the first countries where Lilly launched Mounjaro, and our priority was to bring it to patients as quickly as possible during a time of limited availability.

“At launch, Lilly agreed to a UK list price that is significantly below the European average to prevent delays in NHS availability.

“With changes in the environment and new clinical evidence supporting the value of Mounjaro, we are now aligning the list price more consistently to ensure fair global contributions to the cost of innovation.”

Dr Leyla Hannbeck, head of the Independent Pharmacies Association, said she was “shocked and very disappointed” by the price rise.

“This is a real blow to patients at a time when more and more people, worried about their weight, are turning to jabs.

“It is vital that patients who are already taking Mounjaro talk to their local pharmacy before they take any action.

“Those patients considering weight-loss jabs should also not be put off as Mounjaro is not the only option. Local pharmacies are best placed to offer vital advice about the range of treatments available.”

She said it was important other weight loss drug manufacturers hold their nerve and their prices. Alongside Mourjaro, Novo Nordisk drug, Wegovy, is widely used in the UK.

“The British market for weight-loss jabs is only set to grow, but it will only achieve that growth if treatment remains affordable to the average patient,” Dr Hannbeck added.