Vomiting Multiple Times, Several Hours After Drinking 10 Oz. Of

Vomiting multiple times, several hours after drinking 10 oz. of Magnesium Citrate for constipation - is this serious? Severe constipation from an opioid pain killer after oral surgery (stopped using after 3 days). Today drank Magnesium Citrate after enema would not do the job. Several hours later nausea and vomiting multiple times. Is this something that needs an emergency room, or is it a common side effect? I am finding contradictory answers with a search. Thank you...