Скачать или смотреть Bioavailability- Factors affecting bioavailability (Hinglish)

Bioavailability is defined as the fraction or percentage of an administered dose of a drug that reaches the systemic circulation in its unchanged, active form. It is a vital pharmacokinetic parameter that determines how much and how quickly a drug becomes available in the bloodstream to exert its therapeutic effect. When a drug is administered orally, it undergoes processes such as absorption through the gastrointestinal (GI) tract and metabolism in the liver before entering the systemic circulation. During these steps, part of the drug may be destroyed, metabolized, or excreted, leading to a reduction in the amount that ultimately reaches the blood.

For example, if a patient takes 100 mg of a drug orally, and 30 mg is excreted unchanged, the remaining 70 mg reaches the liver. Out of this, if 30 mg is metabolized during first-pass metabolism, only 50 mg reaches the systemic circulation in an unchanged form. Thus, the bioavailability of this orally administered drug is 50%. In contrast, drugs given by the intravenous (IV) route are delivered directly into the bloodstream, bypassing absorption barriers and first-pass metabolism, thereby achieving 100% bioavailability.

Several factors influence bioavailability, which can be broadly classified into drug-related and patient-related factors. These can be easily remembered using the mnemonic SIMPLE PATIENT.

Drug-related factors (SIMPLE):

S – Solubility: Poorly soluble drugs have reduced absorption and hence low bioavailability.

I – Ionization: Only the unionized (lipid-soluble) form of a drug crosses cell membranes efficiently.

M – Metabolism (First-pass): Extensive hepatic metabolism decreases the amount of unchanged drug reaching the blood.

P – Physicochemical/pH stability: Drugs unstable in acidic pH, such as penicillin G, are degraded in the stomach.

L – Lipid solubility: Highly lipid-soluble drugs cross biological membranes easily, improving bioavailability.

E – Excipients/Formulation factors: Tablet design, coatings, and disintegration rate affect drug release and absorption.


Patient-related factors (PATIENT):

P – Presence of food: Food may enhance or reduce absorption; for example, griseofulvin absorption increases with fatty meals, while tetracycline absorption decreases with milk.

A – Age and genetic factors: These influence metabolic enzyme activity and drug processing.

T – Time of gastric emptying: Delayed emptying or rapid intestinal transit reduces absorption time.

I – Intestinal health: Conditions like diarrhea or malabsorption decrease absorption.

E – Enzyme activity and liver function: Liver diseases or enzyme inhibition can alter drug metabolism.

N – Nature of gastrointestinal pH: Affects the ionization and stability of the drug.

T – Total first-pass metabolism: The higher the first-pass effect, the lower the bioavailability.


In summary, bioavailability reflects how efficiently a drug reaches the systemic circulation and is influenced by both drug properties and patient factors. Drugs with high bioavailability produce effective therapeutic actions at lower doses, whereas drugs with low bioavailability may require higher doses or alternative routes of administration. A clear understanding of these factors helps clinicians optimize dosage regimens, enhance therapeutic outcomes, and ensure consistent patient responses.