A woman has been refused bail after being charged over the alleged repeated poisoning of her young son at a home in Sydney's south-west.

In January detectives from the NSW crime command's child abuse and sex crimes squad commenced an investigation after an 18-month-old boy was treated for a near-fatal overdose of a prescription medication.

Police were told the child was admitted to Sydney Children's Hospital in January with high levels of Tegretol in his system.

Tegretol is a medication used primarily in the treatment of epilepsy and neuropathic pain.

The boy was not prescribed the medication at that time.

A 20-year-old woman was arrested at a home at Lurnea on Wednesday and taken to Liverpool Police Station and charged with seven counts of using poison to endanger life or inflict grievous bodily harm.

The woman is accused of poisoning her young son.

Police allege the woman administered the drug to the child without a current prescription and contrary to medical instruction.

The child has been released from hospital and is in the care of relatives.

Detective Chief Inspector Bryan Parker said it would not be known for a number of years if the child had suffered any neurological damage from the incident.

Police allege the mother lied about the child suffering from epileptic fits to initially gain a prescription for the drug.

Outside the court her solicitor Ted Bramble said the boy was "doing alright".

"She loves him very much," Mr Bramble said. "I hope one day they will be reconciled."

The case returns to court on June 13.