Fears of bad ecstasy batch after four hospitalised in Sydney's south

The three men and a woman are believed to have taken an illicit drug, sold as ecstasy (MDMA), which was thought to have contained a hallucinogenic substance.

Police believe the four ingested the drug in the form of a small blue pill marked with a "scissors" logo, obtained at house parties and a licensed premise, all in the Cronulla area.

"The individuals presented severely agitated as an effect of taking the drug," acting inspector Peter McMaugh said. "It was described by medical staff as being symptoms not associated with ecstasy."

The four people did not know each other, Mr McMaugh said, giving police "great concern" about a dangerous batch of ecstasy being sold in the community.

"They seem to have taken it in separate incidents in situatons not involved with one another."

The four were taken to Sutherland Hospital; three were since been released, but a 20-year-old man remained in hospital under observation, police said. Local officers have commenced an investigation into the matter.

It comes two weeks after 19-year-old Georgina Bartter died at the Harbourlife music festival after reportedly taking one-and-a-half ecstasy pills. The odds of such an acute and irreversible allergic reaction to the drug was one in 10 million, staff at St Vincent's Hospital said.

Ecstasy is Australia's second most popular illicit drug after cannibis. The most recent National Drug Strategy Household Survey found just 3.4 per cent of respondents viewed it as a substance of concern.

Mr McMaugh urged people to be cautious about ingesting substances where they could not be sure of the contents.

Also on Saturday, the drug squad was in the process of dismantling a "large-scale clandestine drug laboratory" on Sydney's Northern Beaches.

Detectives seized more than 200kg of chemicals believed to be used in the manufacturing of MDMA, the pure form of ecstasy.

The Warriewood laboratory was discovered following an explosion and subsequent fire on Friday.