Lynne Massey-Davis, 65, from East Yorkshire, says trying to find Mounjaro stock has been "stressful" after her last prescription order with an online provider wasn't delivered.
"I'm in a holding queue," she says. "It's a very uncertain time. I've spent a lot of time on the phone."
She's been told there will be a two-week delay on delivery. In the meantime, she paid £349 to another provider who then said they too had run out, which she describes as "unethical".
Fifteen months ago when Lynne started taking the weight-loss drug, she had a BMI of 32. Now it's down to 26 and she "feels 10 years younger", regularly doing park runs, going swimming and walking her dogs.
"I'm worried about my health but I will stay safe. There may be many others who will not."
Lynne is planning to share a friend's doses until her order arrives.
The National Pharmacy Association (NPA), which represents 60% of community pharmacies in the UK (around 6,500), said there had been "an increase in demand" for Mounjaro ahead of the planned September price hike.
Eli Lilly has announced the drug's price will rise by up to 170%, meaning a month's supply of the highest dose will go up from £122 to £330.
The company said the drug had been sold in the UK at a price that was "significantly below" that charged in other European countries. US President Donald Trump had previously complained about the high cost of drugs in the US.
"This increase in the cost of Mounjaro has caused understandable concern to patients and pharmacies alike and has resulted in short term pressures on supply," said NPA chief executive Henry Gregg.
He urged patients to avoid bulk ordering Mounjaro because "it can have a significant impact on supply and pose a potential risk to patient safety".
"We'd also urge patients to avoid unlicensed sellers, who may be selling fake medication or medication that does not meet UK safety standards," he added.
Instead, he encouraged patients to speak to their pharmacies for advice.