How are medications absorbed into the body?

Medication Absorption Pathways

Medications are primarily absorbed in the proximal small intestine (duodenum and jejunum) following disintegration and dissolution processes that begin in the stomach. 1 This absorption process is influenced by various physiological, pharmaceutical, and chemical factors that can significantly impact drug bioavailability and therapeutic outcomes.

Routes of Medication Administration and Absorption

Oral Administration

• Disintegration and dissolution: Solid dosage forms (tablets, capsules) must undergo disintegration and dissolution in the stomach, duodenum, and proximal jejunum before absorption 1
• Primary absorption site: Most oral medications are absorbed within the proximal jejunum 1, 2
• Absorption process: After dissolution, drugs cross intestinal epithelium through:
  • Passive diffusion (lipophilic compounds)
  • Active transport via carrier proteins
  • Paracellular transport (small water-soluble molecules)

Parenteral Administration

• Intravenous: Provides 100% bioavailability by bypassing absorption barriers
• Intramuscular/Subcutaneous: Absorption occurs through capillary networks at injection sites
• Transdermal: Absorption through skin layers via passive diffusion 1

Other Routes

• Pulmonary: Rapid absorption through large surface area of alveoli
• Tracheostomy/Endotracheal: Limited to emergency medications (epinephrine, naloxone, atropine, lidocaine) 1
• Intravitreal: Direct injection into eye vitreous with diffusion through hydrated cross-linked meshwork 1

Factors Affecting Medication Absorption

Physiological Factors

1. pH of absorption site:

   • Weakly acidic drugs: Better absorbed in acidic environment
   • Weakly basic drugs: Better absorbed in alkaline environment 2

2. Gastrointestinal transit time:

   • Faster transit: Reduced absorption time
   • Slower transit: Increased absorption opportunity 2

3. Blood flow to absorption site:

   • Increased blood flow enhances absorption rate
   • Decreased blood flow (shock, heart failure) reduces absorption

4. Surface area available for absorption:

   • Small intestine has large surface area (villi and microvilli)
   • Conditions reducing surface area (inflammatory bowel disease, celiac disease) decrease absorption

Pharmaceutical Factors

1. Dosage form:

   • Solutions and suspensions: Faster absorption than solid forms
   • Sustained/delayed-release: Should be avoided in patients with short bowel syndrome 1

2. Particle size:

   • Smaller particles: Larger surface area, faster dissolution and absorption
   • Nanoemulsions: Enhanced absorption for lipophilic drugs 3

Chemical Properties of Drugs

1. Lipophilicity:

   • Lipophilic drugs: Better passive diffusion across cell membranes
   • Hydrophilic drugs: Limited passive diffusion, may require transporters 2

2. Molecular size:

   • Smaller molecules: Easier absorption
   • Large molecules (proteins, peptides): Limited absorption, may require specialized delivery systems

3. Ionization state:

   • Non-ionized forms: Better absorbed through lipid membranes
   • Ionized forms: Poor absorption through membranes 2

Drug-Drug and Drug-Food Interactions Affecting Absorption

Drug-Drug Interactions

• Chelation: Formation of non-absorbable complexes (e.g., tetracyclines with calcium, iron)
• Adsorption: Physical binding of drugs (e.g., cholestyramine binding bile acids) 1
• pH alterations: Antacids changing stomach pH affecting drug dissolution
• Altered GI motility: Drugs affecting transit time 4

Drug-Food Interactions

• Delayed gastric emptying: Food can delay absorption of some drugs
• Enhanced dissolution: Some drugs show increased bioavailability with food
• Reduced presystemic clearance: Food may increase bioavailability of drugs like propranolol 4

Special Considerations

Patients with Short Bowel Syndrome

• Solid dosage forms may have unpredictable absorption
• Liquid formulations preferred
• Sustained/delayed-release medications should be avoided 1
• Alternative routes (IV, topical) should be considered when appropriate 1

Patients with Postpyloric Feeding Tubes

• Some medications require gastric acid for absorption
• Certain drugs may bind extensively to feeding tubes
• Absorption may be altered when bypassing the stomach 5

Switching from IV to Oral Therapy

• Consider bioavailability of oral formulation
• Ensure patient can absorb oral medications
• Not appropriate for critically ill patients or those with absorption issues 6

Monitoring Drug Absorption

• Therapeutic drug monitoring when available
• Clinical response assessment
• Adjustment of dosage form, route, or frequency based on response 1

Understanding these principles of medication absorption is crucial for optimizing drug therapy and ensuring appropriate therapeutic outcomes while minimizing adverse effects.