Drugs do not have a single mechanism by which they exert their effects. A drug is defined as a chemical that disrupts a biological system. This definition implies that a drug is capable of exerting its effect via several different mechanisms. 

For example, a drug may exert its effects by its capacity to denature proteins (commonly observed with astringents), by its ability to act as a membrane surfactant (as seen in amphotericin B), or by chemical interactions with other molecules driven by a drug’s acidic or basic properties (as seen in antacids or Protamine sulfate, an antagonist of heparin). 

Themajority of drugs exert their therapeutic effects by binding to specific receptor sites. Receptors possess two crucial characteristics that help: 

1. They exhibit a relatively high affinity for binding drugs (ligands)
2. Upon binding a drug, receptors initiate a signal cascade that leads to a biological response. 

Many drug receptors are remarkably efficient at transducing biological signals even at very low concentrations.