Understanding Drug Interactions: Effects, Antagonism, and Absorption
Understanding Drug Interactions
General Effects of Drug Interactions
In many instances, drugs or substances are administered concomitantly with the aim of achieving a better therapeutic effect through interaction. An example of this search for beneficial effects through interaction mechanisms is the use of antagonists to displace a toxin from a receptor and reverse poisoning. However, the consequences of the interaction can be harmful or toxic if they render a necessary drug ineffective.
Types of Antagonism
There are different types of interactions, including competitive antagonism, uncompetitive antagonism, and physiological antagonism:
- Competitive antagonism: Agonist and antagonist compete for the same receptor binding site. An increased concentration of one of them shifts the balance to its favor.
- Noncompetitive antagonism: The antagonist binds to the same site as the agonist, but in an area related to it and necessary for the agonist to exert its action. The antagonistic effect cannot be reversed by increasing the dose of the agonist.
- Physiological antagonism: This occurs when drugs have different receptors and opposite actions. That is, they do not compete for binding with the same receptor but bind to different receptors and activate them, producing opposite effects.
Antagonism and Synergism
Antagonism occurs when the action of a drug is decreased or abolished as a result of the presence of another drug acting on the same receptor.
Synergism is the opposite of antagonism; it involves an increase in drug action when administered together with another drug. The effect of drugs used simultaneously is greater than the sum of their individual effects.
Clinical Significance in Drug Absorption
Interactions at the absorption level modify how much of a particular drug is absorbed, resulting in a greater or lesser amount than originally planned, and/or absorption that is faster or slower than anticipated. Changing the amount of drug absorbed is equivalent to administering a dose that is greater or less than needed.
Factors Affecting Digestive Absorption of Drugs
Several factors can affect the digestive absorption of drugs due to interactions with other substances:
- Modification of gastric pH: At a practical level, pH only affects the absorption or elimination of drugs with pH values between 3 and 10.5. An increase in gastric pH means that if the drug is acidic, the polarized form predominates, leading to poorer absorption and better elimination. If the drug is basic, the nonpolar form predominates, resulting in better absorption and poorer elimination.
- Adsorption and chelation: Adsorption refers to the selective accumulation of particles on a surface. Some substances have the ability to “attract” molecules to their surface and generate a new compound, whose physical and chemical characteristics prevent it from being absorbed in the intestine.
- Changes in gastrointestinal motility: Most drugs are absorbed in the duodenum and jejunum. Therefore, a change in peristalsis, whether increased or decreased, can cause unpredictable effects.
- Presence of some nutrients and foods: The intake of nutrients and foods can alter the absorption of drugs, sometimes facilitating it and sometimes decreasing it.