What would you do if you were being slowly poisoned in doses so small it was not easily found by a doctor so no one believed you?

If I thought that was the case then I'd try to consider how that might be happening and by what means. If the poison is in the water or in drink or food then those can be tested. The air can be tested. There is no such thing as an untraceable poison in reality. It's just about degrees of difficulty.