Toxicology
Many chemicals that we call poisons are only toxic when made into different chemicals by the body.  Many chemicals are made toxic in the liver by enzymes, for example paracetamol, which is a common drug. There is also some variation in liver enzymes (caused by genetic variation) in different people that can cause a substance to be more toxic in one person than it is in another. Some substances are also only toxic when they are with other chemicals, as one liver enzyme can cause activity in another enzyme.