Police have issued a warning over the safety of several batches of illegal drugs being sold in Sydney as ecstasy.

Two recent seizures of tables have raised even more concern than normal because they contained dangerous levels of the chemical 25l-NBOMe.

The potent hallucinogen has been blamed for numerous deaths both in Australia and overseas.

25l-NBOMe was the primary ingredient in a tablet ingested by Sydney teenager Henry Kwan shortly before he jumped to his death in June.

The two large table seizures were made in separate raids at Lidcombe in July and Bella Vista earlier this month.

Police have issued a photograph of the seized drugs along with the warning to steer clear.

The Commander of the New South Wales Drug Squad, Detective Superintendent Nick Bingham, says it is a reminder of the often unknown risk of drugs.

"251-NBOMe, which mimics the effects of LSD, can have a shocking impact on a person's mental state, sparking bizarre and dangerous behaviour that has resulted in people being killed," Detective Superintendent Bingham said.

"There's no quality control in the production of illicit and synthetic drugs.

"The people who purchased these tablets would have been expecting an ecstasy-like effect.

"However, based on our testing, the effect would have been far more hallucinatory and could very well have resulted in severe psychotic episodes, leading to reckless behaviour and self-harm.

"Anyone who is considering experimenting with illicit or synthetic drugs should stop to consider the possible consequences and not do it. It's a gamble with your life that's just not worth taking."

251-NBOMe was only declared an illegal substance in September.