Oh, this sounds tough, mama. An absolute non-negoitable causing a daily power struggle. Power struggles effect the overall relationship between parent and child and I imagine as time goes along, it gets worse and worse. Hang in there!

The only thing I can offer is to remember that this is about power. She doesn't have it here and that is "hard to swallow" so to speak. OTOH, is there something else in your lives where you could extend some power to her? If she was able to regain or obtain power in some other aspect of her life, it might at the very least, create a place for daily healing and reconnection between you and her.
Play is a powerful tool toward this end. I suggest a special "playtime" with her each day that is labled as such. A time when you focus on her, undistracted and undivided, and most importantly,
allow her to lead the play: follow her cues and allow yourself to be bossed around, play what she wants to play how she wants to play it for say, 20-30 minutes or as long as you can manage (or stand!

). Maybe immediately following medicine time? I would not present this as a "reward" (I'm not a rewards person I'll admit) but simply because you love her and want to heal the relationship and break the tension and the arching effects that a daily power struggle can cause a relationship. Soley for the purpose of reconnecting the two of you, and giving her a forum to possibly play through this daily trauma. It may or may not eleviate the power struggle itself, but it will go far allowing you both to heal from it and put your relationship in better standing... giving less power to the power struggle itself.
Some possible scenarios that I've used are the obvious approach, in that you make a crown of paper, place it atop her head and tell the "queen" that you are her humble servant. Tell her that for the next 20 minutes, you are hers to boss around in any way she likes. And then do it. Jump in, be silly (or deadly serious!), fall all over yourself trying to keep up with the demands, etc. Another approach would be to simply say, "For the next half hour, I'm all yours! We're playing what you want to play, how you want to play it. Where do you want me?" The key is, to allow her absolute power (baring any unhealthy or life-threatening acts that is

) and let her KNOW that she is the boss in a very plain and clear way. You OTOH, are in the position of the person
without the power. You're likely to see some illuminating things in her play... play is always soooo illuminating if we're tuned in. Maybe she'll want to "feed you the medicine" for a change, or maybe she'll cover this territory in a more subtle way.
I actually try to make special playtime with my son as often as possible whether there are power struggles or not. I believe that this kind of play can actually circumvent the very power struggles that threaten to decay our relationships with our children.
One other thing that has happened with us is that when there is a power struggle going on, DS might take to "creating" more of them in other areas to get his point across. Maybe he'd dump his food more say... if whatever "it" was wasn't actually all that damaging (albeit a little inconvenient), I'd make sure to notice and make a big silly fuss... ACK! You crunched crackers into the carpet! ACK ACK! He thinks, "Mom doesn't want me doing this, but she doesn't have the power to stop me either!" The issue with this approach however is that you have to be in for the "long haul" as it may persist awhile until he's filled up. If you're not up for dealing with it on a daily basis for a good long while, I suggest the "circumvent" route and just play play play!
My two cents. For what it's worth. Hang in there mama!

Em