![]() ![]() Other factors that also impact drug absorption include the following: In addition to a drug’s ionization status, a drug is more likely to be absorbed if it contains lipophilic chemical groups, lacks bulky/oxygenated side chains, and is not too large in size. As a result, acidic drugs are most likely to be absorbed in the acidic areas of the proximal duodenum whereas, basic drugs will be best absorbed in more alkaline areas of the distal ileum. In contrast, the small intestines have a large surface area available for absorption owing to extensive villi and microvilli. The stomach is more of a “storage” organ than an absorptive one. Physiogically speaking, even though the stomach is acidic, it is not well-suited for drug absorption, even for drugs that are weak acids due to its thick mucus layer and relatively small surface area. Practically speaking, this means that if taken orally, a drug that is a weak acid will be absorbed primarily in the acidic environment whereas, a drug that is a weak base will be absorbed in the alkaline environment small intestines. Ionized (or charged) drugs are not absorbed as efficiently as un-ionized drugs are. Thus, the take-home message, again, is that weakly basic drugs are more likely to absorbed in alkaline environments where they remain un-ionized compared to acidic environments where whey gain a proton and pick up a charge. On the other hand, in an alkaline environment, a drug that is a weak base would remain un-ionized. ![]() In an acidic environment, H +, the drug gains a proton and becomes ionized. The opposite is true for drugs that are weak bases. Thus, a key take-home point is: a drug that is a weak acid will be best absorbed in an acidic environment (because it gains a proton and becomes un-ionized). be absorbed) compared to drugs that are ionized. If the same acid, A -, is placed into an alkaline environment, -OH, the drug will remain ionized, as A -.ĭrugs that are un-ionized will be better able to diffuse through a lipid cellular membrane, cross a biologic barrier, and enter the bloodstream (e.g. In the above example, when the weak acid, A -, is placed into an acidic environment, H + the drug picks up a proton and is no longer ionized (it becomes protonated as HA). Drugs that are weak acids will pick up a proton when placed in an acidic environment and will, thus, be un-ionized. Most drugs are either weak acids or weak bases. This type of drug movement does not require any energy expenditure, but will be influenced by the size of the drug and the solubility of the drug.įor drug absorption to be most efficient, the properties of the drug itself and the pH of the environment where the drug is located must be considered. On the other hand, most drugs cross cellular barriers via passive diffusion that is, drugs simply move from an area of higher concentration to an area of lower concentration by diffusing through cell membranes. ![]() Only a few drugs move across cellular barriers in an “active” way that is, a way that requires energy (ATP) and moves the drug from an area of low concentration to an area of higher concentration. Note, that if a drug is administered intravenously (placed directly into the blood stream), the need for absorption is bypassed entirely.įor drug absorption to occur, a drug must cross biologic barriers (e.g. orally, rectally, intra-muscularly, subcutaneously, inhalation, topically, etc.). Absorption can be accomplished by administering the drug in a variety of different ways (e.g. Absorption is the process of delivering a drug into the blood stream. ![]()
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