Once in the body and depending on the instructions from your DNA, drugs have different ways of influencing us.  Drugs are developed to influence the activities of certain cells.  Often this is done by affecting what is called the receptor portion of a cell.  A receptor is a part of the cell that receives a stimulus that prompts a particular action of the cell.  Most antidepressant drugs, like Zoloft or Lexapro, affect the areas of the cells that regulate seratonin, a neurotransmitter (a carrier of instructions) that is thought to affect mood, that will affect the normal action of the cell.

Drugs work in the body based on their affinity, intrinsic activity and dissociation.

AFFINITY

Most of us think of affinity as a liking or identification with something or someone.  When speaking about drugs, affinity refers to both the likelihood of a chemical reaction taking place between the drug and its receptor (the part of the cell that receives instructions), and the strength with which the drug binds to its receptor.  If there are several similar drugs in the brain, the one with the higher affinity will be the one that binds with the receptor and also be the most difficult for other drugs to displace.

INTRINSIC ACTIVITY

As used here, intrinsic means an essential part of something.  Activity means an action.  Intrinsic activity means the extent to which the drug activates or stimulates a receptor.  Drug A can have a high level of intrinsic activity once it is bound to a receptor, but if drug B has a higher affinity and a lower intrinsic activity level, in most cases drug B will bind to the receptor and prevent drug A from ever reaching the receptor.

DISSOCIATION

The word associate means to be with someone or to connect something in your mind with something else.  As used here, the prefix “dis” means to undo.  Dissociation then is the undoing of an association.  Once a drug has attached to a receptor, dissociation is the length of time it takes for a drug to disengage from its receptor.

Drug A could have a higher level of intrinsic activity and take a longer length of time to dissociate from a receptor than drug B, but if drug B has a higher affinity for the receptor, then it will be the one that takes over the receptor.