When cough and cold medicines are taken as directed, they safely treat symptoms caused by colds and flu. But when taken in higher quantities or when you don't have any symptoms, they may affect the brain in ways very similar to illegal drugs, and can even lead to addiction.
All drugs, including cough and cold medicines, change the way the brain works by changing the way nerve cells communicate. Nerve cells, called neurons, send messages to each other by releasing chemicals called neurotransmitters. These neurotransmitters attach to molecules on neurons called receptors. Drugs affect this signaling process. (Learn more about how neurotransmitters work.)
DXM acts on the same brain cell receptors as hallucinogenic drugs like ketamine or PCP. A single high dose of DXM can cause hallucinations (imagined experiences that seem real). Ketamine and PCP are called "dissociative" drugs, which means they make you feel separated from your body or your environment, and they twist the way you think or feel about something or someone.
Codeine attaches to the same cell receptors as opioids like heroin. High doses of promethazine-codeine cough syrup can produce a high similar to that produced by other opioid drugs. Over time, it takes more and more of the drug to get that good feeling. This is how addiction starts.
Both codeine and promethazine slow down activities in the central nervous system (brain and spinal cord), which produces calming effects.
Learn more about how the brain works and what happens when a person uses drugs.