I used to get sick flying transpac to the Philippines. I started using Airborne gummy lozenges starting a week before and throughout the journey in the mid 2000's. Ended up not being sick except the one year I could not find the lozenges. Coincidence? I think not.
I don't like the fizzy nature the drops give like alka seltzer, and will not use them. Now, I use the chewables or hard drops that are available. Working in the airport transport industry, I have found the Increased Vitamin C helps me from getting sick when dealing with hundreds of customers a month. I also keep hand sanitizer available to sanitize my hands when touching surfaces that may be touched by sick people. I would suggest bringing some aboard that meets the allowable TSA Liquid regulations.