PlayMore tests will be carried out on a two-year-old boy who died from a suspected methadone overdose.
Brandon Davis was taken to Birmingham Children's Hospital along with two other children after apparently swallowing some of the drug, used as a substitute for heroin.
Doctors were unable to save Brandon, but a post-mortem examination into his death has proved inconclusive.
Police say further tests will be done to establish the cause of his death.
A three-year-old boy and a boy aged four months are still in hospital after receiving treatment for methadone poisoning.
The older boy is in a stable condition while the baby is being kept under observation as a precaution.
Police are consulting with Birmingham Social Services over care for the boys once they are discharged from hospital.
A spokesman for the hospital said: "We are still treating the other two children who were brought in at the same time. The matter is being handled by the social services."
Two men aged 30 and 33 were arrested after officers were called to an address in Walkers Heath by West Midlands Ambulance Service on March 11.
The 30-year-old man was arrested on suspicion of neglect and the 33-year-old on suspicion of possession of a controlled drug.
Police have not said whether the arrested men are related to the children.
Methadone is an opiate similar to heroin and is prescribed to addicts to help wean them off the drug.
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