There are a wide variety of reasons why a patient collapses, or falls ill, and it is often good detective work by doctors which is critical in saving life.

A number of people arriving at accident and emergency have suffered overdose, whether intentional or inadvertent, and using prescription or illegal drugs.

It may be hard to tell the difference between someone passing out as a result of alcohol poisoning or a collapse caused by drugs or even a head injury - particularly if no-one has seen it happen, and there are no obvious signs of physical injury.

Mr Donald MacKechnie, a consultant in accident and emergency medicine at Rochdale Infirmary in Greater Manchester, says: "Usually there's external evidence if someone has a head injury - but A&E staff are usually relying on information from relatives or whoever brought the person in."

Pathologists at district general hospitals have a limited number of tests they can run, and are normally restricted to spotting paracetamol or Aspirin.

If the person finding the patient can find the source of the collapse, such as an empty bottle of pills, this could be invaluable to doctors, who can start the correct treatment regime immediately rather than having to wait for lab results.

A commonly-used overdose drug is the painkiller paracetamol, which, far from causing a speedy and painless lapse into unconsciouness, can leave patients who survive permanently disabled.

High doses cause excessive liver toxicity, and can lead to liver damage or even failure in surviving patients.

The full effects of a paracetamol dose may not be known until some days after the pills are taken.

Drug overdose

Overdose is an ever-present risk for heroin users, and "epidemics" can be caused by a sudden supply of unusually pure heroin.

However, overdoses with opiates alone can be treated, with a drug called Nalorphine brings a victim of opiate overdose out of his stupor or coma within a few minutes.