Dextromethorphan is a cough suppresent found in most over the counter cough medications. As its intended purposes are a bit questionable, it sure does produce a great high (one of my favorites actually.)
DO NOT take DXM with serotonin antogonists such as MDMA, Cocaine and Benzos. Doing so will MOST LIKEY cause Serotonin Syndrome. Which is a very annoying and fatal disease.
If you're going to overdose on this stuff make sure it's the only main ingredient in the product (flipping the package over and reading the Drug Information takes mere seconds) because the others are more hazordous than rewarding.
Stick to low doses if you're inexperienced, I started with 300 mg and that dose still has me amazed years after.
Take them on an empty stomache, 4 hours of fasting should do it.
Taking some anti-histamines about an hour prior has been reported to combat the nausea but I find smoking just a small amount of cannabis 1 hour after dosing deminishes (decreases) the negative effects completely. Not to mention the perfect synergy they share.
Recreationally dosing on DXM too often isn't the greatest idea, as a rule of thumb you should wait 2 weeks before every trip (even longer would be better).
I wouldn't take DXM without access to cannabis, ths nausea just ruins the experience.
Please, do some research before diving head first into any new substance. Better safe than sorry.
If you have any more questions don't be afraid to ask, I will gladly share my knowledge.