Allergy sufferers have been advised to rely on out-of-date EpiPens amid a global shortage of the life-saving adrenaline injectors.
Those affected have been told to use the device past their normal expiry date, with campaigners urging people not to dispose of it until they have a new one.
The medicines regulator has taken the measure of certifying batches of expired EpiPens as safe, amid fears those going into anaphylactic shock might otherwise be left with nothing.
The body said it has agreed to extend the use-by date of some EpiPens by four months, with pharmacies now rationing them and GPs being forced to switch people to alternative brands.
EpiPens, which make up three quarters of the 340,000 adrenaline injectors prescribed annually, are in short supply because of manufacturing problems in the US.
Pfizer, which makes the product, has said it is "working tirelessly to increase production".
The UK government has said it is working with them to resolve the issue.
EpiPens are prescribed to people with serious allergies to products such as nuts, milk, fish, shellfish, eggs and some fruits.
The shortage comes after an inquest into the death of Natasha Ednan-Laperouse, 15, who suffered an allergic reaction to sesame seeds in a Pret A Manger baguette.
A coroner found the chain had failed to address the seriousness of food allergies.
Sufferers are advised to keep two EpiPens with them at all times in case of anaphylaxis – a severe and sometimes life-threatening reaction often caused by allergies.
The devices can only be used once and usually have an expiry period that lasts for at least a year.
Others brands such as Jext and Emerade are also in shortly supply amid a surge in demand.
Lynne Regent, chief executive of the Anaphylaxis Campaign, has said people shouldn't get rid of their out-of-date EpiPen until they have acquired a replacement.
She added that injectors expire at the end of the month stated on the device, not the beginning.
Ms Regent told The Times: "We are saying don't get rid of the old pen until you have the new one in your hands."
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She added: "Provided the liquid is clear it is fine to use.
"If you were not able to get a new pen, which is unlikely, it is better to have one that's slightly out of date."