Pharm Basics

Pharmacokinetics (ADME)

Pharmacology Basics

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Pharmacokinetics is what the BODY does to the DRUG. Remember the acronym ADME — each phase determines how much drug reaches its target and for how long.

A

Absorption

Movement of the drug from the site of administration into the bloodstream.

Affected by

  • Route (IV = 100% bioavailability, oral = variable)
  • Blood flow to absorption site
  • First-pass metabolism through the liver
  • GI pH, food, and motility
Clinical pearl: Oral drugs lose potency to first-pass metabolism — IV bypasses this entirely.
D

Distribution

Movement of the drug from the bloodstream into body tissues and target sites.

Affected by

  • Tissue perfusion (brain, heart, liver, kidneys first)
  • Protein binding (mostly albumin) — only free drug is active
  • Lipid solubility (lipid-soluble drugs cross the blood–brain barrier)
  • Body composition (fat vs. lean mass)
Clinical pearl: Low albumin (malnutrition, liver disease) → more free drug → toxicity risk.
M

Metabolism

Chemical alteration of the drug, mostly in the liver via cytochrome P450 enzymes.

Affected by

  • Liver function (impairment slows metabolism)
  • CYP450 inducers ↓ drug levels (rifampin, phenytoin, carbamazepine)
  • CYP450 inhibitors ↑ drug levels (grapefruit juice, erythromycin, fluconazole)
  • Genetic variations (fast vs. slow metabolizers)
Clinical pearl: Always check liver function and look for CYP450 interactions before starting therapy.
E

Excretion

Elimination of the drug from the body — primarily by the kidneys.

Affected by

  • Renal function (GFR, creatinine clearance)
  • Other routes: bile, lungs, sweat, breast milk
  • Half-life determines dosing interval
  • Steady state reached after ~4–5 half-lives
Clinical pearl: Renal impairment → drug accumulation → toxicity. Adjust doses based on creatinine clearance.

Key Pearls

  • Half-life = time for serum concentration to drop by 50%
  • Steady state ≈ 4–5 half-lives (for both reaching and eliminating a drug)
  • IV route has 100% bioavailability — no absorption phase
  • Older adults: ↓ liver/kidney function, ↑ body fat → altered ADME
  • Neonates have immature liver enzymes and reduced renal clearance

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