German dermatologist Dr Uwe Wollina made the suggestion after spotting the rash on patients who had been admitted suffering ecstasy-related side-effects.

He said the rash, which appears on the face and neck, could indicate early on which drug users are likely to suffer ill effects.

Her findings were published in the journal Dermatology and reported in New Scientist magazine.

Liver failure

Dr Wollina, a dermatologist at the University of Jena, first noticed the spots on two patients who had taken the drug.

One patient had liver failure and the second was treated for a drug-induced psychosis.

The rash later disappeared after the patients were treated and the drug had left their systems.

Dr Wollina thinks the cause could be the drugs effect on chemicals in the brain - particularly serotonin, which regulates mood, memory, pain, sleep and sex reactions in the body.

He said: "The underlying mechanism may be in the serotonin pathways."

Ecstasy prevents nerve cells absorbing serotonin, leading to high concentrations of the chemical in the brain and other tissue.

Dr Wollina suggests these levels of serotonin increase first the flow of blood to the face and then the activity of sebaceous glands.

When blocked, these glands can produce spots.

Stress reaction

The rash could indicate that the ecstasy user's body is under stress and is far more prone to other serious side effects.

This would be the case even if they are not heavy users, because "the side effects are not dose-dependent," Dr Wollina told New Scientist.

Ecstasy, or MDMA, is a hybrid of the hallucinogenic mescaline and the stimulant amphetamine.

It is one of the most popular recreational drugs, and first came to prominence as the drug of choice of ravers.