This is often condensed to: "The dose makes the poison" or in Latin, "Sola dosis facit venenum".
It means that a substance can produce the harmful effect associated with its toxic properties only if it reaches a susceptible biological system within the body in a high enough concentration (i.e., dose).
[2] The principle relies on the finding that all chemicals—even water and oxygen—can be toxic if too much is eaten, drunk, or absorbed.
"The toxicity of any particular chemical depends on many factors, including the extent to which it enters an individual’s body.
[3] The idea also describes the phenomenon in which poisonous substances can be medicinal in small doses.