A 19-year-old man has died after a suspected consumption of the drug MDMA in St Lawrence.
One woman and three men between the ages of 17 and 26 were arrested and have been released on police bail in relation to drug supply offences.
The police say that they responded to a call by the ambulance service at around 2:30am on Saturday morning (15 February).
On the evening of Saturday night, a 20-year-old man was arrested in St Helier and further drugs and cash have been seized.
He remains in police custody.
Jersey Police say three other males have been treated in hospital for what is believed to have also been from the effects of MDMA consumption. They are said to have now been discharged.
The force say that a number of addresses are currently being searched.
Anyone who may have take MDMA are being advised against the use of the drug and if they have taken it recently and feel unwell or have suffered any side effects to seek medical attention.
Those with information are being asked to call the police on 01534 612 612 or the independent charity Crimestoppers on 0800 555 111.