ST. AUGUSTINE (CBSMiami/AP) — A Florida grandmother was in handcuffs last week following an investigation into the death of her young grandson.

The St. Johns County Sheriff’s Office arrested 59-year-old Brett Aitken last week for the December death of her 15-month-old grandson in St. Augustine.

Mughsot for 59-year-old Brett Aitken. (Source: St. Johns County Sheriff’s Office)

Aitken has been charged with manslaughter after authorities say she accidentally gave the child a fatal dose of methadone.

According to investigators, Aitken kept the controlled drug after its prescribed user died. She placed the liquid in an empty bottle of her grandson’s allergy medicine, telling detectives it was “for a rainy day.”

She told detectives she forgot when the boy had an allergy attack and gave him a lethal dose. The boy’s name wasn’t released.

Methadone helps addicts quit using heroin and other opiates by reducing withdrawal symptoms.

Aitken is jailed on $100,000 bond. Her attorney didn’t respond to an email Sunday seeking comment.

(© Copyright 2019 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)