A sudden death and a hospitalisation in Merthyr Tydfil are thought to be linked to a "dangerous batch" of drugs.
Police have said enquiries suggest both may have been caused by a dangerous batch of street valium circulating in the area, though the death is currently being treated as unexplained.
South Wales Police is now warning people about the risks and urging users who feel unwell to seek urgent medical attention.
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A spokesman for the force said: "Following a sudden death in Merthyr Tydfil, which is currently being treated as unexplained, and one linked hospitalisation, initial enquiries point to a particularly dangerous batch of street valium which could be in circulation in Merthyr Tydfil.
"As a precaution, to prevent further incident, we are reminding users about the risks. People should contact drug support agencies for further advice. Users should seek immediate medical attention if they are feeling unwell."
Anyone with information about the supply of illegal valium should call 101 and give reference *289655.
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