The deaths of four people in the space of a day are believed to be linked to heroin, police have said.
Three men and a woman were found dead at four separate addresses in the Barnsley area on Good Friday.
South Yorkshire Police said it is not linking the deaths but warned drug users to exercise caution.
Temporary Chf Insp Ian Proffitt said the force is investigating in the deaths are linked to the strength or content of heroin being sold locally.
He said: "We are very concerned. For four deaths to occur in similar circumstances in a small time period and in a relatively small geographical area is unusual.
“We are currently exploring to establish if they are linked to the strength or content of heroin being used locally.
“The public should exercise caution if they come into contact with controlled drugs, particularly heroin, or heroin derivatives.
"If you experience any unusual symptoms after taking drugs, seek medical attention immediately."
Two men, aged 33 and 40, were found dead at separate addresses in Barnsley on Friday morning, while a 47-year-old woman was found dead at a house in Grimethorpe.
A third man, a 31-year-old, was found dead at an address in Barnsley on Friday evening.
A coroner's investigation is underway to establish the cause of each death and the force is awaiting the results of post mortems and toxicology tests.
A 37-year old man and a 42-year-old man have been arrested on suspicion of being concerned in the supply of controlled drugs and released on bail.
Anyone with information about the supply of drugs is urged to call South Yorkshire Police on 101, quoting incident number 164 of April 14 or call Crimestoppers anonymously on 0800 555 111.