What you are abusing is dextromethorphan, a cough suppressant that in large quantities acts like LSD. It is incredibly dangerous, and you risk death or brain damage every single time you take it.
There are no know physical withdrawal symptoms except nausea, restless legs, insomnia, etc.
Please, I beg of you, for the sake of those who love you, stop doing this. When my daughter was in high school (2006) we were at a concert at her school one night when the drummer collapsed and went into violent seizures. He had been abusing the Coricidin HPB Cough and Cold pills.
Our local pharmacy started putting them behind the counter.
BTW, that kid didn't die, but he has never been the same. He was a gifted musician...now he works as a janitor at some bar. He looks awful every time i see him. Yes, I'm hoping I scare you.
I wouldn't say fatal unless you have some serious underlying health problems that you haven't mentioned. It also depends on the dose you were taking in this case. If you would to share a bit more with us we can help you and support you.