An investigation is underway after two deaths this week were linked to a new drug similar to ecstasy.
The incidents took place in Dublin and the South East of the country. The HSE said a number of other deaths in April were also being investigated.
The tablets are green in colour with an apple or Rolex symbol on them.
The HSE is also warning of the dangers of other new stimulants, which are believed to be a combination of cocaine and amphetamines.
Consultant in emergency medicine at Sligo Regional Hospital Dr Fergal Hickey said: "These are killing young, healthy people…These are perfectly healthy people who think they're going to get a buzz from something, and end up dead."