A Pennsylvania mum has been charged with criminal homicide for the death of her 11-week-old son after an autopsy showed the baby had died because of drugs in the mother's breast milk, prosecutors said.

Samantha Jones, 30, told police she had been too tired to make the baby a bottle when he awoke crying and instead breastfed him.

She then went back to sleep but awoke to find the infant was pale and had bloody mucous coming from his nose.

She called 911 and was instructed to perform CPR on the child after he went into cardiac arrest.

Police later arrived on the scene and accompanied the child to hospital but was pronounced dead at 8.29am on April 2.

The Bucks County District Attorney’s office said a post-mortem examination found the baby died from a combination of methadone, amphetamine and methamphetamine poisoning transmitted through Jones' breast milk, according to the charging documents.

Jones told police that she had been prescribed methadone to help her overcome her addiction to painkillers, and took it during pregnancy and after giving birth, according to a statement from Buck's County District Attorney's Office dated July 13.

Jones' attorney Louis Busico said the baby's death was not intentional, according to the district attorney's office statement.

Jones has now been released on a $3m bail and a preliminary hearing is scheduled for Aug. 29, the district attorney's office said.

She has been instructed to stay away from anyone under the age of 18. She has a 2-year-old son who is in the custody of his father.

The case is being investigated by the New Britain Township Police Department and the Bucks County Detectives.