The advice given to CCF cadets regarding eating wild plants is, as I recall:

  • Rub a little on the the legs. The feet have thicker skin but if they become inflamed then you can't walk anywhere. Wait twelve hours. If there is no reaction, proceed to the next stage.
  • Rub the potential food on the wrists. This puts any poison present closer to the bloodstream, providing a more sensitive test. If there is still no reaction, proceed.
  • Put a little of the food on the tongue. Wait another twelve hours, for any reaction. If there is still no reaction, eat up!
You'll notice that this whole process takes up to 36 hours before you can actually eat anything. It is designed for you to begin as soon as you find yourself in a foreign environment, rather than when the food supplies are exhausted and the troops are starving.

The system is in typical British Army style: damage limitation. It'd have to be a pretty strong poison to take a man down just from contact with thick skin, and then the sensitivity of the tests are built up from there. The only risk would be if the test subject built up his own resistance, while the other troops would dive straight in after the trial period. That said, I don't know of any cases in which this occured.