This compared with 31 of the 79 from July 2017 to June 2018 and just 13 of the 87 between July 2016 and June 2017.
The figures were released as part of inquiries for a Rotorua Daily Post story in which a father spoke out after his teenage son overdosed on nine prescription tramadol pills another student gave him at school.
At the time, the father said he was shocked when an emergency department nurse told him prescription drug abuse was widespread among high schools.
"To hear it's just glossed over by some kids saying 'everyone's doing it' was also a shock."
At the time, city high school principals were asked if the use of prescription drugs to get high at schools was an issue.
John Paul College principal Patrick Walsh and Rotorua Girls' High School principal Ally Gibbons said they had been told about the issue while Rotorua Boys' High School principal Chris Grinter said news of the issue had come as a "major shock".
Lakes DHB also operates Taupō Hospital and provided figures for patients who presented at the Taupō emergency department.
In Taupō, four of the 13 patients who presented at the emergency department due to drug overdose between July and September this year, were 18 and under.
This compared with 13 of the 41 between July 2017 and June 2018 and three of the 40 between July 2016 and June 2017.
Dr Tania Pinfold, the clinical leader of youth health at Rotovegas Youth Health, said much had been made of the drug tramadol in recent weeks but all prescription medication was dangerous in the wrong hands.
"They are many potentially abusable drugs and whether it's tramadol or something else is irrelevant. It's important for all prescription medications to be kept safely.
"Experimentation is a huge part of adolescents so we need to make sure there are fewer opportunities for disasters such as making prescription medications accessible when they shouldn't be ... If drugs are not there, they can't do dumb things with them."
Patients presented at Rotorua ED due to drug overdose
• July 2018 to September 2018: 21 patients in total, 11 (52 per cent) aged 18 and under.
• July 2017 to June 2018: 79 patients in total, 31 patients (39 per cent) aged 18 and under.
• July 2016 to June 2017: 87 patients in total, 13 patients (15 per cent) aged 18 and under.
• July 2015 to June 2016: 80 patients in total, 15 patients (19 per cent) aged 18 and under.
Patients presented at Taupō ED due to drug overdose
• July 2018 to September 2018: 13 patients in total, 4 patients (30 per cent) aged 18 and under.
• July 2017 to June 2018: 41 patients in total, 13 patients (32 per cent) aged 18 and under.
• July 2016 to June 2017: 40 patients in total, 3 patients (1 per cent) aged 18 and under.
• July 2015 to June 2016: 51 patients in total, 11 patients or (22 per cent) aged 18 and under.