A spate of drug-related deaths has prompted a warning about contaminated heroin in Yorkshire and Teesside.
Four people died in Barnsley, South Yorkshire, on Friday, with two men dying the following day in Leeds and Normanton in West Yorkshire.
Recent testing on recovered drugs found traces of the strong prescription painkiller fentanyl.
Police said it was too early to connect the deaths to fentanyl, but warned drug users to be "extremely cautious".
In March, a warning was issued by Cleveland Police after six recent deaths connected to a batch of low grade heroin in Stockton-upon-Tees.
An "unusually high" number of deaths connected to drug use in Hull was reported in February, but none confirmed as of yet where fentanyl has been present.
The painkiller, which hit the headlines after it was linked to the death of US singer Prince, is considered to be 50 times more potent than heroin according to America's Drug Enforcement Agency (DEA).
What is fentanyl?
Fentanyl is an extremely strong painkiller, prescribed for severe chronic pain, or breakthrough pain which does not respond to regular painkillers.
It is an opioid painkiller which means it works by mimicking the body's natural painkillers, called endorphins, which block pain messages to the brain.
The risk of harm is higher if the wrong dose or strength is used.
Typical symptoms of a fentanyl overdose include slow and difficult breathing, nausea and vomiting, dizziness and increased blood pressure.
Det Supt Nick Wallen, of West Yorkshire Police, said: "We are however urging those people who regularly use Class A drugs and particularly those who purchase their drugs via street suppliers to be extremely cautious in relation to what they are taking.
"Anyone experiencing any unusual symptoms after taking drugs should seek immediate medical attention."