You don't need medication! Plus you have no idea what the reaction will be and the last place you want to be is trapped on an airplane with an emergency!
Benadryl will be the most common recommendation from friends and family. But, there are a couple things to be aware of. First, it will knock your little one out in most cases, but as soon as it wears off they will wake up feeling miserable and drugged. The other issue is that at any time you give it to your child, instead of sedating, it can suddenly create a reaction of hyperactivity. The complete opposite of what you are hoping for!
We have flown to Panama with little ones and never used anything. We try to keep to our usual schedules of napping and sleeping, if you are still nursing, try to nurse during take-off and landing to keep your daughter calm and help her ears. Otherwise a pacifier or bottle will help too.
Bring toys and distractions and plan on a lot of play and no sleep for you. If she sleeps, try to sleep at the same time to reduce your jetlag. If you are on an overnight flight, then you will be fine, she'll get tired and sleep on her own once the novelty wears off.
They have short attention spans, so as oftern as possible, get up and walk the aisles with her. People don't mind seeing happy babies and the like a smile. Plus when food or drink service comes down the aisle you will be trapped for a bit.
One last thing, put your bag above head and keep a smaller diaper bag with everything you need for a couple of changes handy along with snacks, drinks and toys for your daughter handy. Makes life a lot easier! Don't forget to layer her in comfortable clothes and bring a small blanket. The temperature fluctuates tremendously on planes and it helps to be able to keep her comfortable to keep fussiness down.
Enjoy your trip!