I Took Alka Seltzer Plus Sinus Congestion +pain Around 5:15pm

I took alka seltzer plus sinus congestion +pain around 5:15pm tonight while I was out to dinner to help with symptoms I began experiencing while out. I have developed a cough and want to take my promethazine dm oral solution. How long should I wait until its safe to take it since the alka seltzer plus has dextromethorphan in it?