Deputies arrested the toddler's 51-year-old grandmother, Cynthia Watson, of Oregon, Monday morning in Garner Valley after she tested positive for methamphetamine, according to the Riverside County Sheriff's Department.
Earlier that day, authorities said the toddler's mother was trying to put her to bed Sunday night and noticed she was acting strange — scratching her skin, talking rapidly, appearing agitated and unable to sit still.
Authorities subsequently launched an investigation and discovered Watson was staying with the toddler and her mother.
On Sunday night, she had apparently made herself a cup of tea with methamphetamine to get high, authorities said.
Deputies believe Watson left the tea unattended, at which point the toddler likely took a sip from the cup, authorities said.
Watson was later taken into custody on suspicion of child endangerment and possessing methamphetamine.
Detectives are still investigating the case to determine how the child had access to the tea and whether there are any previous incidents of endangerment.
"It's a complicated process," Martinez said.