People with severe allergies have been told they can use their EpiPens past their normal expiry date, amid a continuing shortage of the devices.
The medicines regulator said it had agreed to extend the use-by date of some 0.3mg EpiPens by four months.
It comes as parents told the BBC their children's supplies were due to expire in a few days, and they had been unable to get replacements.
The government says it is working with EpiPen's makers to resolve the issue.
EpiPens are the most common adrenaline auto-injectors (AAIs) and are prescribed to people with potentially serious allergies, such as to nuts, milk, fish, shellfish, eggs and some fruits.
People are recommended to keep two of the devices with them at all times for cases of anaphylaxis – a severe and sometimes life-threatening reaction often caused by allergies – as one is not always enough.
They can only be used once and usually have an expiry date that lasts for a minimum of 12 months.
There have been shortages of EpiPens in the UK and other countries for months.
There has also been a shortage of the alternative AAIs prescribed in the UK – Jext and Emerade – amid a surge in demand for them.
In response to the shortages, the Medicines and Healthcare Products Regulatory Agency (MHRA) said it had agreed to a request by Mylan, the company behind EpiPen, to extend the expiry date of some batches of the 0.3mg version of the device.
The MHRA said it made the decision after it was given evidence that showed the shelf life of the 0.3mg pen was 20 months from the date it was manufactured.
It said the devices would work "just as effectively" during the extended expiry period.
But the extension does not apply to the 0.15mg version, given to children under 30kg.
EpiPens should not be used if the solution is discoloured, brown, there are solid particles, or signs of leakage or damage.
Other AAIs should never be used after their expiry date without first checking with your medical practitioner.
'At my wits' end'
Lynne Riles said she had been trying to get an EpiPen prescription for her seven-year-old daughter for the past six weeks.
"The pens we have now expire at the end of September, and I just feel at my wits' end."
Sal Smith said her four-year-old's EpiPens were also due to expire at the end of the month, after she had tried for four weeks to get more.
"Still no sign of them… I'm getting worried now."
What can cause a severe allergic reaction?
- food – such as peanuts, milk/dairy foods, egg, wheat, fish and seafood, sesame and soya
- insect stings – from bees and wasps
- medication – such as antibiotics and ibuprofen
- latex – rubber gloves, balloons, swimming caps
Mylan has no stock of the 0.15mg pens left, the Department of Health and Social Care said in guidance to healthcare professionals.
More of these pens are due in October but this is not expected to meet normal demand, the department said.
Emma Cummings, whose 10-year-old son Loui has severe allergies, said if supply problems continued and she had no AAIs left that were in date, she would be forced to keep her son indoors, as he is at risk from airborne allergens.
"Loui wouldn't be allowed out of the four walls of our own house, where I can categorically state that there is nothing in the house to trigger any allergies for him.
"That will restrict his education, his after-school activities, but you would.
"Parents such as ourselves, with a child like Loui, would have no choice."
The DHSC guidance said that if people were left with nothing but expired AAIs they should keep them – until they get replacements – and be prepared to use them.
While AAIs lose their potency and become less effective after their expiry date, they are not harmful, and it is better to use an expired AAI than none at all, it said.
It said at present it was believed that "careful management" of existing supplies would prevent the need for using expired pens.
Health minister Lord O'Shaughnessy told the BBC: "We are doing everything we can to ensure patients continue to access the medications they need, and we have issued detailed guidance to healthcare professionals."
Mylan said Pfizer (which manufactures the drug) was "working hard" to increase production, and supply would stabilise between October and December this year.