Allergy patients are being warned of a potential fault with Emerade adrenaline pens.
The Medicines and Healthcare products Regulatory Agency (MHRA) said some have blocked needles, so cannot deliver adrenaline.
Around two in every 1,000 pens are thought to be affected.
Patients are advised to follow the existing advice to carry two pens at all times. Charities said it was a “very difficult” time for patients.