Two canisters of nitrous oxide have been stolen from a storage cupboard in the grounds of Alnwick Infirmary.
The theft of the gas, more commonly known as laughing gas, took place between 5pm on Friday and 7.30am on Saturday.
Thieves broke the security chain and lock to the cupboard to break-in and steal the canisters.
Acting Inspector Mick Quinn said: “The gas has a similar effect to helium and if it is consumed in large amounts, it has the potential to suffocate the user.
“It is also a flammable source if it is exposed to a naked flame or extreme heat conditions.
“I’d ask anyone who has information about the theft to contact police immediately.”
Officers are carrying out inquiries and keen for any witnesses or anyone with information about the offence to get in contact by calling 101 – extension 69191, quoting reference 376 020815 – or Crimestoppers anonymously on 0800 555111.