Leann Louise Gross, 38, is charged with endangering the welfare of children, possession of a small amount of marijuana and possession of drug paraphernalia.
Pennsylvania state troopers said they were called to a residence along Lincoln Avenue in White Township around noon on Thursday.
Before police arrived, the child was taken to a hospital for treatment of an injury that was not thought to be life-threatening.
Troopers said Gross was watching the child and had left him out of her sight for an undetermined amount of time.
During that time, troopers said the child found the vaping device, which belonged to Gross, from a bedroom nightstand.
Troopers said Gross found her grandson standing next to the nightstand with "smoke" around him and the vape pen on the floor next to his feet. That is when Gross called 911.
After troopers gathered this information and spoke with medical staff, they found out the child displayed indicators of being under the influence of THC.
A search warrant was executed on the home around 2:22 p.m. Thursday and police seized an EVOD vaping device and related items, according to investigators.
Troopers said they are still trying to determine how the child was able to use the device.
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