A Sydney mother has been accused of repeatedly poisoning her young son and causing a near-fatal overdose.

Detectives from the State Crime Command's Child Abuse Squad began investigations in January after an 18-month-old boy from the city's west arrived at the Sydney Children's Hospital suffering from a serious overdose.

The toddler was found to have high levels of Tegretol in his system.

Tegretol, which is mainly used to treat epilepsy and neuropathic pain, was not prescribed for the little boy.

Police will allege in court that his 20-year-old mother was giving him the unprescribed drugs in the face of medical advice to the contrary.