SAN FRANCISCO (KRON) – San Francisco health officials are warning the public on Thursday that teens using marijuana could develop brain damage.

This comes as San Francisco prepares for the legal sale of adult-use pot in January.

“With the loosening of restrictions for adults, and the expected surge in cannabis businesses and advertising, it is crucial that teenagers know the facts,” said Barbara Garcia, who is the San Francisco Health Director. “Young people are smart. We need to support them with clear information about the new law, the risks of cannabis use and how to withstand the influence of targeted advertising.”

In Nov. 2016, California voters approved Proposition 64, which made non-medical pot available for sale. Only those who are 21 years old and older can use pot recreationally.

Officials say 71 percent of San Francisco high school students have never used cannabis, which is lower than the national average of 59 percent, according to a 2015 study by the National Drug Early Warning System.

“Using cannabis is not something that every teenager does, despite the myths and messages to the contrary,” Garcia said. “We’d like to keep it that way and support youth in their decision-making. We want to make sure they know that cannabis is still illegal if you’re under 21.”

Doctors suggest waiting until your mid-20s to start using pot.

“Delaying cannabis consumption is the smart thing you can do for your brain, which is still developing into your 20s,” San Francisco Health Officer Dr. Tomas Aragon. “While you are young, cannabis can harm your memory and ability to learn and pay attention. It also impairs driving, and you can get a DUI by driving high.”

Officials say other possible dangers of teens using pot include the increased risk of respiratory illness and decreased motivation and memory. This can inhibit youth from reaching their goals as they grow into adulthood, officials said