Officers were called to a property on Union Street at around 2.45pm on Bank Holiday Monday to reports of a sudden death of a 42-year-old man.
Initial enquiries into the death have led officers to believe it could be linked to the illegal use of the drug amphetamine.
A second male, aged 55, remains in hospital in a critical but stable condition said West Yorkshire Police.
Det Insp Mark Catney, of Kirklees CID, said: “We want to remind people of the dangers of taking illegal drugs. This is a tragic incident which has resulted in the death of a 42-year-old man.
“We want to urge people to reconsider before taking substances illegally – the dangers of taking drugs are well known and all drugs pose a risk.
“People who take these drugs don’t always know what’s in them, the effects they may have or where they have come from.
“We believe this particular death may be linked to illegally taking the drug amphetamine. If anyone has taken amphetamine and has fallen ill, they should seek medical attention immediately.”
Anyone with information on the illegal use of drugs should contact police on 101 or independent charity Crimestoppers on 0800 555 111.