The teen was arrested for failing to appear in court on robbery charges but while in custody admitted he'd been smoking methamphetamine and huffing petrol.
A police spokeswoman said the officers grew concerned for the teenager's health so he was taken to Middlemore on Tuesday last week, with a police escort, for assessment.
But when he arrived, he was agitated and "severely aggressive", said Dr Vanessa Thornton, clinical director of Emergency Care at Middlemore.
Hospital staff sedated him for his own and others' safety.
But he woke abruptly, assaulted the police officer and during a struggle stole the pepper spray.
A man whose aunt was in Middlemore at the time of the incident said the teen ran around the emergency department and his aunt was among several people sprayed in the face. The man was soon detained by police and hospital security.
Dr Thornton said several people suffered eye irritation and coughing. The hospital could not say how many were affected.
Reactions to pepper spray can include a burning sensation, difficulty breathing and coughing.
Pepper spray can also cause death in combination with medical conditions like asthma.
Dr Thornton said the hospital was not carrying out its own investigation because it was "very apparent" what had happened.
The teen was remanded in custody after being charged with aggravated assault, assault with a weapon and escaping custody.
