Hi,
The likelyhood would be the player suffering minor fatigue and the onset of symptoms, but that they never come to fruition. Give them a scare, make them think they are ill. You know that feeling, when you are convinced you have a cold or flu, but then nothing happens.
In the case of a poison, it would be a queasy feeling, a short lived but worrying pain, but nothing more.
Perhaps the character drops a fatigue level for a short duration.
Trust me I am a nurse.
Simon