The action of drugs on the human body (or any other organism's body) is called pharmacodynamics, and the body's response to drugs is called pharmacokinetics. The drugs that enter an individual tend to stimulate certain receptors, ion channels, act on enzymes or transport proteins. As a result, they cause the human body to react in a specific way.
Based on drug action on receptors, there are 2 different types of drugs:
Agonists – they stimulate and activate the receptors
Antagonists – they disable the agonists from stimulating the receptors
Once the receptors are activated, they either trigger a particular response directly on the body, or they trigger the release of hormones and/or other endogenous drugs in the body to stimulate a particular response.
Drugs interact with receptors by bonding at specific binding sites. Most receptors are made up of proteins, and the drugs can therefore interact with the amino acids to change the conformation of the receptor proteins.
These interactions are very basic, just like that of other chemical bondings:
Mainly occur through attractions between opposite charges; for example, between protonated amino (on salbutamol) or quaternary ammonium (e.g. acetylcholine), and the dissociated carboxylic acid group. Similarly, the dissociated carboxylic acid group on the drug can bind with amino groups on the receptor.
This type of bond is very strong, and varies with the inverse of the distance between the atoms so that it can act over large distances.
Cation-π interactions can also be classified as ionic bonding. This type of interaction occurs when a cation, e.g. acetylcholine, interacts with the negative π bonds on an aromatic group of the receptor.
Ion-dipole and dipole-dipole bonds have similar interactions, but are more complicated and are weaker than ionic bonds.
There is a small but significant attraction between hydrogen atoms and polar functional groups (e.g. the hydroxyl [-OH] group). These so-called hydrogen bonds only act over short distances, and are dependent on the correct alignment between functional groups.