Absorption of Drugs

Absorption of drugs

Absorption is defined as the movement of drugs or medications from its site of administration into the bloodstream. The rate of absorption is just as significant as the portion of the administered dose that is absorbed. The medication must pass through biological membranes, unless it is administered intravenously. Absorption is governed by the above-mentioned principles.

Factors affecting absorption of drug

Aqueous solubility

Solid drugs must dissolve in the aqueous phase before absorption. For drugs with low water solubility, the rate of dissolution determines the rate of absorption. For example, Aspirin. Medication or drug given as watery solutions is absorbed faster compared to solids form or oily solutions.


Passive diffusion relies on concentration gradients, with concentrated solutions absorbing drugs faster than dilute solutions.

Area of absorbing surface

The larger the surface area, the faster the absorption.

Vascularity of the absorbing surface

Blood circulation removes drugs from the absorption site and maintains a concentration gradient across the surface. Increased blood flow speeds up drug absorption, similar to how wind speeds up drying clothes.

Route of administration

Drug absorption varies depending on the route of administration due to unique factors. This includes,


The lipoidal gastrointestinal tract epithelial lining acts as an effective barrier to drugs taken orally. Drugs that are lipid soluble and nonionized are easily absorbed from the stomach and intestine at rates that correspond to their lipid: water partition coefficient. Basic medications are mostly ionized and are only absorbed after they reach the duodenum, whereas acidic drugs are primarily unionized in the acid gastric juice and are absorbed from the stomach.

Absorption of drugs from stomach is slower even for the acidic drugs because of the thick mucosa and small surface area. Surface area for absorption in small intestine is much more compared to stomach due to presence of villi, hence gastric emptying can accelerate the rate of absorption of drugs. Presence of food also affect the absorption of certain medications. Therefore, most of the drugs are absorbed better if taken empty stomach.

Some drugs have low oral absorption because the gut epithelium-resident efflux transporter P-gp extrudes some of the absorbed drug back into the intestinal lumen. This process contributes to the low oral bioavailability of cyclosporine and digoxin.

Subcutaneous and Intramuscular

The drug is applied subcutaneously or intramuscularly, right next to the capillaries. Drugs that are soluble in lipids easily cross the entire surface of the capillary endothelium. Even large lipid insoluble molecules or ions can be absorbed without obstruction by capillaries with large paracellular spaces. Through the lymphatic system, very large molecules are absorbed.

As a result, many medications that are not absorbed orally are absorbed parenterally. While absorption from the subcutaneous site is slower than that from the intramuscular site, overall, both types of absorption are quicker, more reliable, and more consistent than oral absorption. Heat application and physical activity increase blood flow, which speeds up drug absorption.

Topical sites

This includes skin, cornea and mucous membranes. Lipid solubility of drug is the primary factor that determines systemic absorption following topical application. Nevertheless, very few medications actually penetrate intact skin. For example, Hyoscine, Fentanyl etc. The drug incorporated in an oleaginous base can be rubbed in order to enhance absorption, or you can use an occlusive dressing to hydrate the skin.

The cornea is permeable to unionized, lipid-soluble physostigmine but not to highly ionized neostigmine. Drugs administered as eye drops have the potential to be absorbed through the nasolacrimal duct; for example, timolol eye drops have the potential to trigger asthma attacks and cause bradycardia.

 Lipophilic drugs are absorbed by the mucous membranes of the mouth, the vagina, and the rectum. Therefore, vaginally applied oestrogen cream has caused gynecomastia in the male partner.


The term “bioavailability” defined as the rate and the degree of drug absorption from a dosage form as shown by the drug’s excretion in urine or by its concentration-time curve in blood. It is a measurement of the fraction (F) of a drug’s administered dose that enters the bloodstream unaltered. When administered intravenously, a drug’s bioavailability is 100%; however, this often decreases when taken orally.


Drugs in oral formulations from various producers, or batches from the same manufacturer, may contain the same amount of the substance (chemically equivalent), but they may not produce blood levels that are biologically equivalent. When, under appropriate test conditions, the rate and extent of the active drug’s bioavailability from two drug preparations do not differ significantly, those preparations are considered bioequivalent.


Drug absorption is a critical process influenced by various factors. Physicochemical properties, pharmaceutical formulation, and patient-related aspects all play a role. Lipophilic drugs tend to be absorbed more efficiently. Factors like route of administration, gastrointestinal motility, blood flow, liver metabolism, disease states, age, and gender impact absorption. Bioavailability, which reflects how much of a drug reaches its target site, is directly affected by these factors. Understanding drug absorption is essential for optimizing therapeutic outcomes.

Frequently asked questions

Define Absorption of drug?

Absorption is defined as the movement of drugs or medications from its site of administration into the bloodstream.

What are the factors affecting absorption of drugs?

Aqueous solubility, concentration, route of administration etc. are the factors that affects absorption of drugs.

What is bioavailability?

Bioavailability is defined as the rate and the degree of drug absorption from a dosage form as shown by the drug’s excretion in urine or by its concentration-time curve in blood.

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