Officers issued the warning after being made aware of a possible batch of particularly strong drugs – believed to be blue diazepam tablets – being circulated in the Southwick area of Sunderland.
An investigation was launched after a woman in her 30s sadly died and two other people became unwell in recent weeks.
Police say that after carrying out a number of enquiries, it is suspected that they had all taken the drugs.
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Northumbria Police said a large quantity of the illegal drugs was seized by police and a 58-year-old man was arrested on suspicion of being concerned in the supply of drugs.
Officers say he has since been released under investigation.
Detective Chief Inspector Sean McGuigan has warned anyone in possession of such tablets not to take them.
“First and foremost, our thoughts go out to the loved ones of the woman who has sadly died and we are continuing to establish the circumstances around her death,” he said.
"Although our investigation is still at a very early stage, we are warning people against taking these tablets.
“Unless you have been properly prescribed medication via your doctor or pharmacy, you can never be sure what substances contain and the impact they could have on you.
“The risks of taking illegal substances are well documented and we would urge people to think about the potential consequences and not to take them.
"It is important that any illegal drugs are disposed of safely and not left in public – and they can be taken to police stations.
“As our investigations continues we would also encourage anyone with information about the drugs in question to contact us at the earliest opportunity.”
Anyone who has information about the drugs should contact Northumbria Police using the ‘Tell Us Something’ page on our website or by calling on 101 and anyone who has taken these drugs and needs emergency medical attention should always call 999.