Principles of pharmacology and pharmacokinetics - absorption

Definition

Drug absorption (movement from the site of administration into systemic circulation) is influenced by formulation, route, gastrointestinal factors (pH, motility, presence of food), and drug physicochemical properties (lipophilicity, molecular size, ionisation state) - each factor representing a potential point of interaction or variability affecting how much and how quickly a drug reaches its target
Relevant to essentially all non-intravenous drug administration, given absorption's central role in determining onset of action and overall drug exposure

Lipophilic, un-ionised drug molecules cross biological membranes more readily via passive diffusion, explaining why a drug's ionisation state (affected by the local pH environment, e.g. gastric vs intestinal pH) substantially influences absorption rate and extent at different points along the gastrointestinal tract

Not directly measured clinically for most drugs, but understanding absorption principles explains clinically relevant interactions - e.g. antacids/calcium-containing products chelating certain antibiotics (tetracyclines, fluoroquinolones) and reducing their absorption if co-administered too closely in time

Timing separation (several hours apart) between interacting drugs affecting absorption (e.g. levothyroxine and calcium/iron supplements, certain antibiotics and antacids) preserves adequate absorption of both agents
Food interactions (some drugs require empty stomach administration for reliable absorption, others require food to reduce GI irritation or enhance absorption) should be specifically counselled

Antacid/mineral-drug chelation interactions (tetracyclines, fluoroquinolones, levothyroxine), food effect on absorption (enhancing or reducing depending on the specific drug), enteral feeding tube drug administration considerations

Levothyroxine absorption is significantly reduced by co-administration with calcium, iron, or antacid products through direct chelation in the gut - this is a frequently encountered, clinically significant interaction managed simply by separating administration timing by several hours, rather than avoiding the interacting medications altogether

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