Police are investigating after six clubbers were taken to hospital with overdoses of the dance drug GHB.
GHB is popular with many clubbers
Three women and three men were taken to hospital in Liverpool after taking the drug, which has also been linked to date rapes.
They were all at Garlands nightclub on Dale Street in the city centre on Saturday night.
Paramedics were first alerted at about 0000 GMT on Sunday when two 18-year-old girls collapsed.
Between 0000 GMT and 0600 GMT a further four people were taken ill at the club.
They were a 21-year-old woman, two 21-year-old men and a 25-year-old man.
All were unconscious and were taken to the Royal Liverpool Hospital for treatment before being discharged on Sunday.
Other club-goers at Garlands were warned about what had happened.
Merseyside Police have launched an investigation and have spoken to people who were at the club.
Officers have also searched the premises.
Increased drugs incidents
They said the six people arrived at the club at different times between 0000 GMT and 0200 GMT.
It is believed they had travelled to Liverpool from outside the city.
A spokesman for the Royal Liverpool Hospital said drug-related incidents were on the increase on Merseyside,
He said: "It is thought that the drug involved last night was a derivative of GHB and Ecstasy.
"All six people have now been discharged but we were very worried at one stage about two of the women.
Long term effects unknown
"They were treated in an intensive care therapy unit.
"When people are brought in unconscious from taking drugs the recovery time is short, but we don't know what the long term effects will be."
A spokeswoman from national drugs organisation Frank said GHB has become increasingly available in recent years.
She said: "GHB is readily available in clubs. Effects can vary widely and when mixed with alcohol it can be fatal.
"As it is odourless and in liquid form it can easily be slipped into drinks,
so people should be careful when they put drinks down."