Parents have been warned to be extra vigilant after an upturned needle was found on swings in Nanny Goat Park in Fingerpost over the weekend.
The horrific find was made by a mum who had brought her two young children to the park.
She went to wipe the swing seats when she came across the needle.
A spokesman for St Helens Council said all its staff had been placed on high alert, with park rangers, cleaners and maintenance staff told to remain extra vigilant.
The authority is also working closely with the police to help apprehend those responsible.
Parents are also been asked to check equipment in plays areas before allowed their children to use it.
Coun Richard McCauley, cabinet member for Transport, Housing and Community Safety, said: “This was an horrific incident and I shudder to think what the potential consequences could have been.
“We are working with the police to apprehend those responsible but in the meantime everyone should take great care – and check play equipment before using it.”
A council spokesman added: “Equipped children’s play area areas within parks are inspected and cleansed on a daily basis throughout the week by council staff - and patrols are carried out at weekends. The council also responds immediately to collect discarded hypodermic needles reported by the public.
“St Helens Council commissions a specialist needle and syringe programme for people who inject drugs with needles, syringes and other injecting equipment.
“This is provided by a number of local community pharmacies and at the specialist community treatment provider at Lincoln House.”
The council also commissions a household medical waste collection service that can be accessed via http://www.sthelens.gov.uk/clinicalwaste
Anyone with any information is urged to call Merseyside Police on 101 or Crimestoppers, anonymously, on 0800 555 111.