This probably makes me a horrible parent, but it works for us--

Our twins (almost 3) started refusing their meds at times a year or so ago and our tactic seems to work. We tell them that if they don’t take their pills than they have to have a shot. Somtimes they test us and we actually have to go to the cabinet and get a syringe and Solu-Cortef bottle and pretend like we are are getting it ready. The thought of it (or the sight of the syringe) makes them gobble their pills right up.

They know that if they don’t take the pills, the shot is coming, so they comply fairly well now.