Absorption of Drugs Through Different Routes:

1. Gastro-intestinal tract:

• Drugs administered by oral routes quickly finds its way to stomach, passing through esophagus.
• Oral mucosa has a thin epithelium and rich vascularity which favours absorption; however, contact time in mouth is usually very short for substantial absorption to take place.
• Effective barrier to orally administered drug is epithelial lining of G.I tract.
• According to pH-partition hypothesis, acidic drugs are best absorbed from stomach and basic drugs are best absorbed from intestine.
• Most of acidic and basic drugs are largely absorbed from the intestine due to larger surface area of intestine, high perfusion rate, long residence time of drug and pH range from 5-7.5.
• Presence of food in G.I tract often dilutes the drug and retards its absorption. Gastrointestinal fluids usually promote disintegration within the ruminal and intestinal lumen by microflora or within mucosa by host enzymes also retards drug absorption and makes certain drugs ineffective orally. Ex; catecholamines.
• Rapid gastric emptying increases drug absorption but rapid intestinal motility decreases the drug absorption.

2. Parenteral sites:

• Drug absorption from intramuscular and subcutaneous sites is rapid as drugs are deposited directly in vicinity of the capillaries.
• Lipid soluble drugs readily pass across the whole surface of the capillary endothelium. Capillaries are highly porous and donot obstruct absorption of even large lipid insoluble or ionized drugs. Thus, drugs which are not absorbed orally are able to reach systemic circulation when given parenterally.
• Aqueous solution of drugs are rapidly absorbed from IM injections sites within 10-30 minutes. Increasing blood supply to IM or SC injection site by application of heat, massage or exercise increases rate of absorption while immobilization of site, local cooling or application of tourniquet retards drug absorption.

3. Pulmonary sites (Alveoli):

• Some drugs intended for systemic effects are administered by inhalation either as gases or aerosols.
• The volatile and gaseous anesthetics are absorbed instantaneously because they have relatively high lipid to water coefficients and generally are small molecules.
• Large surface area of alveoli, high permeability of alveolar epithelium and rich perfusion rate also facilitate rapid absorption.
• In cases of aerosols, drug delivery to lungs is dependent on the size of droplets. Larger particles remain confined to upper respiratory tract and donot reach the pulmonary tree, whereas small particles reach deep into the respiratory tract and cause rapid absorption from alveoli.

4. Topical sites:

• Drugs may be absorbed through skin or mucus membrane to produce systemic effects. Systemic absorption after topical application depends primarily on lipid solubility of drugs.
• Lipid insoluble drugs generally penetrate the skin or mucus membranes poorly.
• Absorption of drugs through skin is usually poor because keratinized epidermis acts as barrier. However, it can be enhanced by rubbing, use of occlusive dressing or rarely by iontophoresis.
• Inflammation, cuts, rashes, mild burns etc. promote drug absorption through skin because these conditions destroy the epidermis.
• Absorption of most drugs through mucus membrane is usually rapid than skin because mucus membrane is thin and highly vascular.
• Systemic absorption through mucus membrane produces unwanted effects however some drugs are applied to facilitate absorption and produce systemic effects.