Is this a domestic flight or an international flight?
If international, check the laws of the country you're going to. The US has one of the strictest medicine regulations in the world; the choice of ingredients is severely limited. Some countries can sell medicine with strong depressants over-the-counter, like Tylenol. Maybe you can take a weak American medication and tough it out on the way there. However, you can buy one of the local medications, take it before getting on the place, and simply relax during the entire flight.
I actually have a personal story regarding this. I was in Israel a while back, and I got sick with a cough. I used a combination of bhand gestures and fragmented Hebrew to ask a store clerk that I needed cough syrop; the sales clerk handed me a bottle. Since the entire label was in Hebrew, I couldn't even read the dosage, let alone the ingredients list. Now spending a 14-hour flight with a cough certainly wasn't the best way to end a trip. So after I got on the plane, I simply took the same dose I would have taken for a similar US-made drug. Well, after about 20 minutes, I felt a Vicodin-like trance flood my brain. Since I was really tired, I quickly fell asleep, and didn't wake up until about 2 hours before landing, and my cough was much weaker then. So why does everyone hate Israel?:roll:
But I digress. If you're going to a different country, buy a local anti-anxiety medication that's better (read: stronger) that the over-the-counter stuff you find here. It should make the flight back a lot more tolerable.