Fallout 76 Antiseptic is heavily used in the making of several narcotic items, and is sought after to make quick healing packs. Although not the MOST needed resource in the game, it seems to be the one you always have on hand due to how often you can find it.
There are several items in the game that can be broken down into Antiseptic:
Abraxo Cleaner
Blood Sacs
Turpentine
Each of these items is incredibly common and you will constantly be finding on your adventures. BUT, the one that is most easily obtained is probably Blood Sacs from fighting Ticks. Ticks have several spawns throughout the map, some guaranteed and some not. Here are some Tick spawn locations to help you quickly scrounge up Antiseptic when you need it most!