Five gas cylinders containing nitrous oxide have been stolen from Dantree Hospital, Daventry.
A store room at the hospital was broken into sometime between 4pm on Tuesday and 5am on Wednesday.
The gas is potentially dangerous if taken in high quantities and could explode if exposed to high temperatures.
It is believed the offenders would have needed a vehicle to remove the cylinders.
Anyone with information is asked to call police on 101 or Crimestoppers anonymously on 0800 555 111.