Tamalpais Union High School District is urging parents to talk with their kids about the effects of the drug, which causes, among other things, hallucinations.
Three students from Tamalpais High were recently sent to the hospital after feeling sick. Two other students from Drake High School were also treated.
School officials now say the students had abused a cold medication called Coricidin. It comes in pill or syrup form.
"It's known as Triple C on the street and at extremes it has the same effect as angel dust which is a hallucinogen," Paul Lofolm of Ross Valley Compounding Pharmacy.
The main ingredient is DXM cough suppressant sold over the counter. People often combine the syrup with a soft drink like Sprite.
Rappers like Lil Wayne have been known to glorify similar, more potent drinks through their music.
"This is new to us, yeah. This year is the first time we've seen this in any kind of numbers," said Tara Taupier of the Tamalpais Union High School District.
A 2014 survey done by the University of Michigan found that 1 in 30 teens reported abusing it to get high.
"In high doses causes a lot of issues including seizures and respiratory depression," Lofolm said.
In rare cases it can cause death.
All five schools within the Tamalpais Union High School District have sent out letters to parents notifying them of the dangers of the drug.
"I was dismayed that this was going on. I had no idea," said Donna Dobryn, a parent.
According to the school district, students typically find the cough medicine at home or have been known to steal it from the store.
It's certainly not new in other counties or cities but school officials in Marin Co. say it's new to them. https://t.co/0wZLmzokfo— Lyanne Melendez (@LyanneMelendez) October 21, 2015
On the streets it's called "Triple C" or "Skittles." https://t.co/z9Ph4hStsR— Lyanne Melendez (@LyanneMelendez) October 21, 2015