How Medications Get Absorbed By Your Body

Main Ideas

Medication absorption is the movement of a drug from its administration site into the bloodstream.

Oral Medications

Oral medications enter the stomach. They may dissolve and pass through the stomach lining, or travel to the small intestine—the most common site of absorption—where they dissolve and cross the intestinal wall.

After absorption from the digestive tract, oral drugs travel through the portal venous system to the liver. In the first-pass effect, the liver metabolizes some of the drug, sometimes inactivating it or sending it into bile for elimination. The active drug that remains enters general circulation and can reach target organs.

Injected Medications

Injected medications bypass absorption in the gastrointestinal tract:

• Intravenous (IV): Enters the bloodstream directly.
• Intramuscular or subcutaneous: Enters muscle or tissue beneath the skin, then passes between cells into capillaries and onward to the circulation.

Bioavailability and Formulations

Bioavailability is the portion of a drug dose that reaches the bloodstream. Oral drugs have less than 100% bioavailability because of the liver’s first-pass effect. IV drugs have 100% bioavailability because they bypass it.

Drug formulations can affect how quickly and how extensively a medication is absorbed. For example:

• Tablets dissolve at different rates.
• Enteric-coated drugs dissolve in the small intestine rather than the stomach.
• Sustained-release formulations contain small spheres that dissolve at different rates, releasing the drug over time. Absorption can therefore vary.

Factors That Affect Drug Absorption

• Dissolution rate: Drugs that dissolve faster are absorbed faster.
• Surface area: The small intestine has more surface area than the stomach lining, so most drugs are absorbed faster there.
• Blood flow: Faster blood flow can maintain a stronger concentration gradient between the drug at the absorption site and drug-free blood, speeding absorption.
• Lipid solubility: Drugs that dissolve readily in lipids pass through the phospholipid cell membrane more easily than drugs with low lipid solubility.
• pH partitioning: Absorption depends on the relationship between pH at the administration site and pH in the plasma, which affects drug ionization and movement into the blood.

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