Professor James Ferguson, a consultant in the emergency department at Aberdeen Royal Infirmary, said that medics doctors were becoming increasingly concerned about the long-term effects of the substances on patients.
Four men in their 30s were admitted to the hospital over the past few weeks with muscle breakdown which could cause tissue to move into the bloodstream, leading to liver or kidney failure and potentially death.
The medical condition is more commonly seen in survivors of road accidents or patients who have been crushed during the collapse of a building.
Prof Ferguson said tests have been carried out on patients admitted to the hospital after taking legal highs.
Enzyme test results of 300 or under would be expected from patients with healthy muscle - but one man returned a result of 167,000 after taking psychoactive substances.
He said: “We are seeing some really ridiculously high levels.
“If you are using these legal highs again and again, you have to know what the long-term effects are going to be. This is a significant workload for us just now and it is a regular workload.
“Obviously I am worried about the people who we see at the hospital.
“But I am also worried about those who use legal highs who we don’t see.”
The surgeon said legal highs – also known as new psychoactive substances (NPS) – were often taken by people with a history of using illegal drugs.
Medics have even been treating patients in their 50s coming into the hospital suffering from the effects of taking unregulated legal highs.
Prof Ferguson said legal high users were being admitted to hospital with varying conditions, including the potentially fatal serotonin syndrome where excessive nerve cell activity can cause confusion, agitation and heart problems.
Some of these patients end up in intensive care and while others are given fluids and observed for a few hours before they are released.
The consultant said he fears that people believe the substances are safe to take because they appear on the shelves in “professional” packets, but do not know how their body will react because they are unregulated.
The products used to be bought online but are now more easily accessed with shops springing up in various locations across the city. Legal highs are often sold as incense, salts or plant food with a label warning that they are not for human consumption.
One store in Aberdeen was last month closed down under a three-month banning order which was the first to be granted by a court in Scotland on a shop selling the substances.
The shop opened up again a few weeks later and the owner has vowed to return to his former store premises once the three-month ban has come to an end.
Nuisance customers were breaking into flats to take NPS products on George Street and harassing shoppers as they walked along the road.
However, the Harminasion shop opened up again a few weeks later on King Street - round the corner from the city police headquarters.
And the owner vowed to return to his former store premises once the three month ban had ended.