PA series of drug overdoses, including one death, has prompted a warning from an emergency doctor about a lethal drug on the streets of Sydney.

St Vincent's Hospital director of emergency, Gordian Fulde, said several people had been admitted to the hospital since Friday, as a result of taking the illegal drug GHB, otherwise known as G.

Dr Fulde said the drug was dangerous, especially mixed with alcohol.

"On Friday night, we got our first one, a lady, there's all ages, but mainly around the 30s," he said.

"And then on Saturday we had four cases, three of which ended up in ICU, the other one needed resuscitation.

"Then it got worse on Sunday, in that we had a DOA or death on arrival."

He said the drug worked like a general anaesthetic.

"It starts making your reality distorted, then you go to sleep, then you become more and more unconscious and then you become even more unconscious and stop breathing and that's what kills you."

A police statement said inquiries were being made but the exact circumstances of the death were unknown.

"Any person who takes any illicit drug is gambling with their life," the statement read.