I got poison resistance even though I used the meds.
Viewing post in Path of Martial Arts (IF) comments
The only way to obtain poison resistance (in the game script) is through the option "Do nothing" (and sending Mei Mei away when she shows up) once the poison triggers. You'll need regen trait and 180 vitality to pass the check. If you buy the medicine earlier (for 500 points) or use the other paths once the outbreak happens you won't enter that script branch.
So, what you describe, is pretty much impossible. Are you sure you didn't confuse two or more of your playthroughs?