The four cylinders, containing litres of nitrous oxide, could raise up to £200,000 after they were taken from a store.
Nitrous oxide - also known as N2O - is often sold to recreational users in balloons in nightclubs, and at festivals.
But health experts warned an overdose can cause death.
Police said the cylinders were stolen from Hairmyres Hospital in East Kilbride.
Two men forced their way into the hospital's cylinder store, and stole the laughing gas, before making off in a dark Audi estate car.
Police Constable Matt McCabe said: "Extensive inquiries are continuing to trace the two men responsible for this theft.
"I would ask anyone who saw two men acting suspiciously, or anyone near ward 18, to contact police."
Laughing gas has become a popular "party high" and is the second most used drug among young people in the UK.
It is understood the gas may have been "stolen for recreational purposes".
Police said there are no descriptions of the suspects following the theft at 10.35am on Monday, May 26.
Health bosses from NHS Lanarkshire warned laughing gas is "dangerous" if misused or consumed without medical supervision.
A spokesman said: "Nitrous oxide or N2O, commonly known as laughing gas, has been known to be used illegally as a recreational drug.
"It dissolves in the bloodstream, depleting the blood of oxygen, and reduces the flow of oxygen to the brain and vital organs.
"Overdoses can be fatal.
"It can also cause a stroke or produce side effects including hallucinations, seizures, blackouts and incontinence.
"We would strongly advise anyone who is offered this gas, or indeed any other gas, not to inhale it, as there is no way of knowing what the effects will be.
"We would also ask anyone who is offered gas to contact the police."
Anyone with any information is asked to contact East Kilbride Police Office on 101.