Drugs and drug targets

I was reading a medicinal chemistry book and came across this statement " If useful drugs can be poisons at high doses or over
long periods of use, does the opposite hold true? Can a poison be a medicine at low doses", can someone elaborate the statement to me please?

Drug targets are a kind of biological macromolecule in the body that have a pharmacodynamics function by interacting with drugs, such as certain proteins and nucleic acids. Drugs achieve disease treatment by binding specific targets and changing gene function of their targets