Thieves cut through a gate then stole several 4ft cylinders filled with laughing gas from a hospital during the early hours.
South Wales Police said the theft from Prince Charles Hospital in Merthyr Tydfil took place between midnight and 6am on Sunday.
Officers believe a vehicle would have been used to remove the "several" stolen canisters of nitrous oxide.
A police statement said: "We are warning the public not to buy or use these cylinders as they could be dangerous."
Cwm Taf University Health Board said the thieves cut off a chain on a gate to gain access to the area where the cylinders were securely stored in the estates department.
Nitrous oxide has various legitimate uses from dentistry to the food industry and in hospitals but it is also known as laughing gas.
Smaller metallic canisters that contain the gas are a common sight across the streets of Wales.
When inhaled the gas can make people feel euphoric and relaxed, according to leading drug advice service Talk to Frank.
The happy feeling some feel led the drug to be nicknamed laughing gas.
Some people can also experience hallucinations.
The relaxing feeling it causes is why dentists use the drug as way of numbing pain.
As well as the numbness and relaxation it often leaves people unable to think straight, causing fits of laughter. For others it can bring on a sudden headache.
The colourless gas is sometimes described as having a "slightly sweet" smell and taste, according to Talk to Frank.
It is normally bought in the small canisters and then transferred to a container, often a balloon, so it can be inhaled.
Talk to Frank said there is a risk of death from using the drug as it can cause a lack of oxygen.
Although the gas has been legal in the past, since the Psychoactive Substances Act came into effect on May 26, 2016, it is now illegal to supply or import nitrous oxide for human consumption.
Anyone with information is asked to call 101 or Crimestoppers on 0800 555 111 quoting occurrence 1900169487.
