After being swallowed, injected, inhaled, or absorbed through the skin, mucosa under the tongue, or mucosa inside the cheek, most drugs enter the bloodstream and circulate throughout the body. (See also Definition of Drug Dynamics.) Some drugs are given directly to the area where they are wanted. For example, eye drops are put directly into the eyes. The drugs then interact with cells or tissues where they produce their intended effects (target sites). This interaction is called selectivity.
Selectivity is the degree to which a drug acts on a given site relative to other sites.
Relatively nonselective drugs
Relatively selective drugsNonopioid Pain Relievers), target any area where inflammation is present.
Highly selective drugs
Where drugs exert their effects has to do with how they interact with specific cells or substances such as enzymes.
Receptors on Cells
On their surface, most cells have many different types of receptors. A receptor is a molecule with a specific 3-dimensional structure, which allows only substances that fit precisely to attach to it—as a key fits in its lock.
Some drugs attach to only one type of receptor. Other drugs, like a master key, can attach to several types of receptors throughout the body. A drug’s selectivity can often be explained by how selectively it attaches to receptors.
A Perfect Fit
A receptor on the cell’s surface has a 3-dimensional structure that allows a specific substance, such as a drug, hormone, or neurotransmitter, to bind to it because the substance has a 3-dimensional structure that perfectly fits the receptor, as a key fits a lock. |
Agonists and antagonists
Drugs that target receptors are classified as agonists or antagonists. Agonist drugs activate, or stimulate, their receptors, triggering a response that increases or decreases the cell’s activity. Antagonist drugs block the access or attachment of the body’s natural agonists, usually neurotransmitters, to their receptors and thereby prevent or reduce cell responses to natural agonists.
epinephrine (adrenaline) and norepinephrine (noradrenaline), which are released during stress. Antagonists such as beta-blockers are most effective when the concentration of the agonist is high in a specific part of the body. Similar to the way a roadblock stops more vehicles during the 5:00 PM rush hour than at 3:00 AM, beta-blockers, given in doses that have little effect on normal heart function, may have a greater effect during sudden surges of hormones released during stress and thereby protect the heart from excess stimulation.
Enzymes
Some drugs target enzymes, which regulate the rate of chemical reactions, instead of attaching to receptors on cellsrifampin, the contraceptive is metabolized (that is, broken down into inactive components) and removed from the body more quickly than usual and may therefore be ineffective.
Chemical Interactions
Some drugs produce effects without changing the function of a cell and without attaching to a receptor. For example, most antacids decrease stomach acid through simple chemical reactions. Antacids are bases that chemically interact with acids to neutralize stomach acid.
