I had constant nausea and vomiting off and off for almost a year in the past. After seeing several doctors, I was finally diagnosed with a stomach bacteria that I probably got from sharing a hookah at a hookah cafe I frequented at the time. After a month's worth of antibiotics and quitting smoking altogether, the nausea and vomiting disappeared completely.
If there is constant vomiting, there has got to be a reason for it. It may take some time to figure out the underlying cause, but this symptom doesn't occur for no reason.