How can a medication influence your dreams?

Any medication you take is going to affect the balance of your brain chemicals, and influence what and how you think if you are sleeping or not. That is after all what meds such as Valium or Prozac are for, to alter your thinking and chemistry on purpose. Don't know why onions in particular would have such an effect, is it possible you are allergic to them? Sometimes an allergic reaction is very subtle... Good luck...