If you're headed on a road trip, it's always wise to have a first-aid kit on you in case of emergency. The Gadling weblog covers what you'll need to pack in your basic medical kit when you're traveling, which includes all the necessities—home medications, band-aids for minor cuts and scrapes, latex gloves for protection, sunscreen, matches, hand santiser, and safety pins and bandannas for quick slings, in addition to other must-have items. All of their suggested items are small enough to be stored within a polycarbonate water bottle which won't get crushed in your luggage.
As Gadling states, your first aid kit should never be left behind as it is an insurance policy that you'll hopefully never have to use. When you do, however, you'll be thankful that you did.