How do I get through to an adolescent that weed is terrible when they continue to smoke it?

KeepTalking with him. Marijuana dependence is the body's physical need for the addictive substance in order for the teen to feel like he is functioning normally. Over time, drug dependence leads to physical harm in the brain and body, to behavior problems, and to association with people who also abuse drugs and commit crimes to obtain drugs. Mj is illegal under U.S. Law, so a teen can end up with a criminal record.
Repeat yourself. Continue to deliver this excellent message, and set an example yourself.
Weed side effects. "smoked marijuana damages the brain, heart, lungs, and immune system. It impairs learning and interferes with memory, perception, and judgment. Smoked marijuana contains cancer-causing compounds and has been implicated in a high percentage of automobile crashes and workplace accidents.".