I am afraid the person with “medical qualifications” is confused about dextromethorphan. It is not an opiate.
It is used widely as a cough suppressant. Previously, other ingredients that were opiates were used in over-the-counter cough medicines, but that is no longer the case in the US. ??The way that dextromethorphan works is by blocking a fast excitatory neurotransmitter, known as glutamate. There are several types of glutamate receptors, and the one that is affected by dextromethorphan is known as an NMDA receptor. In this way, it is similar to a drug of abuse known as ketamine, which has a similar mechanism.
The doses effective at cough suppression and that are present in cough syrup are, of course, much lower than you would need to get “high” from, and I can not personally imagine the idea of drinking 10 or 15 bottles of cough syrup to have a dubiously mild sedative effect, but addicts are what they are. Dextromethorphan has recently been investigated as a potential therapeutic agent in a numerous different and seemingly unrelated fields, ranging from autism, Tourette’s to stroke. The reason for this, to oversimplify, is that in healthy brains excitation and inhibition are balanced. Damage (such as stoke) can cause levels of excitation that can be toxic. Some diseases may, at least in part, manifest some of their symptoms because some pathways controlling certain behaviors are over-excited, or are not inhibited properly. The nerve pathway that controls coughing (via the vagus nerve) actually controls a lot of other involuntary motor behaviors, for example. The hope is that some of the involuntary repetitive behaviors in autism and Tourette’s might be reduced by this treatment.
Maria Gulinello, Ph.D. in Behavioral Neuroscience