Not necessarily. "Medicines safety" is a standard phrase in the world of medicines safety and regulation. We don't know who the sentence "You can win a hundred dollars by conpleting a survey on medicines safety communications." is aimed at. I can't understand the intended situation here. Medicines safety communications go to healthcare professionals - doctors and pharmacists. I can't imagine any of those wasting time on a survey for the chance to win a prize of $100. My hourly rate was more than that.You should use the singular in that sentence.