Life is not risk free : a cancer patient is treated by giving arsenic trioxide AsO3 a well known poison which can lead to multiple organ damage yet recent work have shown it to induce remissions in some leukemias.
Are all substances toxic?
The answer is Yes!!
All are toxic to some quantifiable degree or we can say depends on amount of consumption. Vitamin A for example is necessary for vision, yet it can cause cancer at high dosages.
Signal Words:
'Warning' reflects an intermediate degree of relative toxicity.
'Danger' reflects the highest degree of relative toxicity.
What about risk from chemicals?
Risk evaluation of a chemical is carried out by exposing test animals (usually rats) *which is illegal ofcourse* and then monitoring for signs of harm.
In 1927, J.W. Trevan invented the idea of LD50.
LD50 (Lethal Dose) is the dose which kill 50% of the sample population in a specified period through exposure.
LD50 - measured as the amount of substance per kilogram body weight/mass.
LD50 is essentially helping you into the question how much is too much for a given substance.
In general,
Smaller the LD50 = more toxicity of chemical
Greater the LD50 = less toxicity of chemical
LD50 values of selected Chemicals given below: 
