Receptor Occupation Theory

Research evidence suggested the most of the drugs show specific action through the specific receptor. Receptors are macromolecule, which presents at the cell surface inside the cell and each receptor has a structural specific binding site like a lock for the ligand and the 3D structure of the specific drug molecule (ligand) acts as the key; means stereo-structural specific ligand (drug)-receptor complex (“lock-key model”) may responsible for the biological action of a ligand or drug. Therefore, just like locks and their keys, the interactions between drugs and their targets are highly specific and based on physical shape interactions.

For example-

noradrenaline acts on alfa receptor mainly while isoprenaline acts on beta receptor
levo-noradrenaline is 10 times more potent than dextro- noradrenaline