Detectives investigating after a teenager ended up in a coma - and two were others left critically ill - believe they may have taken homemade 'UPS' pills.
The 'highly dangerous' tablets were discovered at an address in Bury and are being tested.
Police described the tablets as containing 'a concerning number of components', and are warning anyone who may have taken a pill matching the description to urgently seek medical attention.
The 'beige'-coloured pills are said to have a strong, sickly smell and are very fragile, leading police to believe they could be reduced to powder form.
It is understood the boys, two aged 15 and one aged 14, took the unknown substance on Wednesday night.
One of the 15-year-olds and the 14-year-old were taken to hospital and police were called.
Both remain in a serious, but stable condition.
The second 15-year-old was taken to hospital by police after being arrested on suspicion of possession with intent to supply.
He became unwell and is now in an induced coma.
Chief Inspector Phil Spurgeon, from GMP's Bury borough, branded the UPS pills 'highly dangerous', saying that although officers have seen similarly branded tablets before, they could contain completely different substances.
A 40-year-old woman has also been arrested on suspicion of possession with intent to supply and is being quizzed police.
- Anyone with information about the incident should call police on 0161 856 2459, 101 or Crimestoppers on 0800 555 111.
- Anyone who has concerns about drug use can contact Bury Early Break Substance Misuse Service on 0161 723 3880 or the local Alcohol and Drug Service on 0161 253 6488.