Several people have suffered apparent overdoses of the drug GHB at a Melbourne nightclub overnight.

Emergency crews were called to the Inflation nightclub on King Street in the CBD just before 3:00am on Sunday.

Police said four revellers from the nightclub, in their teens and early twenties, were taken to hospital, but it is believed their conditions are not life-threatening.

Officers are now investigating the incident.

What is GHB? Gamma hydroxybutyrate is a synthetic drug usually taken as a colourless, odourless liquid that tastes slightly salty

Gamma hydroxybutyrate is a synthetic drug usually taken as a colourless, odourless liquid that tastes slightly salty Effects including euphoria, increased sex drive and lowered inhibitions can be felt within 20 minutes

Effects including euphoria, increased sex drive and lowered inhibitions can be felt within 20 minutes Also lowers temperature and heart rate, and can cause memory lapses, drowsiness, dizziness and headaches

Also lowers temperature and heart rate, and can cause memory lapses, drowsiness, dizziness and headaches Overdose symptoms include vomiting, sweating, irregular breathing, confusion and agitation, hallucinations, blackouts and seizures

Overdose symptoms include vomiting, sweating, irregular breathing, confusion and agitation, hallucinations, blackouts and seizures Alternatively known as fantasy, grievous bodily harm (GHB), liquid ecstasy or fishies

One witness said a man was detained at the scene.

While not related to ecstasy, GHB (Gamma-hydroxybutyrate) is a popular "party drug" in the dance and club scene and is often referred to as liquid ecstasy.

Ambulance Victoria's Mick Stephenson said overall last night six people in the city were taken to hospital for drug overdoses, some of which occurred outside of venues.

"I don't think last night was out of the ordinary, we have nights frequently where we'd see perhaps ten or 20 overdoses in and around the city," he said.

The Victorian Government has previously conceded that harm reduction efforts to address overdoses had not been effective in the state, following a number of large-scale incidents.

Dozens of revellers were taken to hospital in February after apparently taking the drug at a dance festival at the Sidney Myer Music Bowl.

While in January, three people died after taking a bad batch of ecstasy linked to nightclubs on Chapel Street in South Yarra.

The Government is working to close a legal loophole in the Drugs, Poisons and Controlled Substances Act which would apply a blanket ban to the production, sale and promotion of any substance that has a psychoactive effect.

Currently synthetic drug makers are getting around the legislation by tweaking the formula of products after they are banned, to create "new" products that were not listed on the state's banned register.