How Medications Get Absorbed By Your Body

Medication absorption is the movement of a drug from its site of administration into the blood. Oral drugs enter the stomach where they either dissolve and pass through the cell membranes of epithelial cells lining the stomach or travel undissolved through the stomach to the small intestine, which is the most common site of absorption. Here, drugs dissolve and pass through the intestinal wall. Oral drugs then travel through the portal venous system to the liver, where they undergo the first pass effect. During this process, the liver metabolizes some of the drug, either inactivating it or excreting it into bile for elimination from the body. The remaining amount of active drug leaves the liver and reaches general circulation and target organs. If a drug is administered via intravenous injection, it passes directly into the bloodstream, thus bypassing absorption in the GI tract. If administered through intramuscular or subcutaneous injection, the drug enters either muscle or subcutaneous tissue, where it passes through gaps between cells into capillary walls and then into general circulation or target organs, also bypassing absorption in the GI tract. Bioavailability is the net amount of a dose of a drug that is actually absorbed into the bloodstream. The bioavailability of oral drugs is less than 100% because of the first pass effect of the liver. In contrast, the bioavailability of IV drugs is 100% because they are not exposed to the first pass effect of the liver. Different drug formulations alter bioavailability, because they are not absorbed at the same rate or to the same extent. For example, tablets dissolve at varying rates. Enteric coated drugs dissolve in the small intestine, not the stomach. Because gastric emptying time differs between individuals, absorption times also vary. Sustained release formulas contain tiny spheres that dissolve at different rates, resulting in a steady drug release throughout the day, but also resulting in variable absorption. Some factors affecting drug absorption are the rate of dissolution. Drugs that dissolve faster are absorbed faster. Surface area, since the lining of the small intestine has more surface area than the lining of the stomach, most drugs are absorbed faster in the small intestine. Blood flow, the greater the concentration gradient between the drug-filled stomach and the rapidly flowing drug-free blood, the faster the absorption. Lipid solubility, highly lipid soluble drugs pass through the phospholipids in the cell membrane more easily than drugs of low lipid solubility. And PH partitioning, absorption is faster when the difference between the PH at the site of administration and the PH of the plasma attracts more drug molecules to ionize in the plasma.