The effects of the drug ecstasy cannot be dismissed as minor, according to an intensive care specialist at Auckland Hospital.

A Massey University study shows ecstasy use has doubled since 1998, with men aged 20 to 29 the biggest users.

More than 5,500 people aged 15 to 45 were questioned.

Users said their main problem was exhaustion and depression after dancing all night.

Dr Tony Smith says the potential side effects include death, with at least two fatalities in Auckland - but he says ecstasy users are generally aware of the dangers.

He says there have been at least two deaths in Auckland from ecstasy and another near-fatality last weekend.

He says a number of young people, knowing the dangers, will try dangerous things because they feel they are indestructible and bad things will only happen to someone else.