_____ are chemicals of low molecular masses $(\approx 100-500u)$. These interact with macromolecular targets and produce a biological response.

Drugs are chemicals of low molecular masses $(\approx 100–500u)$. 

These interact with macromolecular targets and produce a biological response. 

When the biological response is therapeutic and useful, these chemicals are called medicines and are used in diagnosis, prevention and treatment of diseases. 

If taken in doses higher than those recommended, most of the drugs used as medicines are potential poisons.

Hence the correct option is A.