Biliary/Fecal: Excreted into bile, then feces.

Other: Lung (volatile anesthetics), sweat, milk.

Half-life ($t_{1/2}$): Time for plasma drug concentration to reduce by half. $t_{1/2} = \frac{0.693 \times V_d}{\text{Clearance}}$.

Clearance (CL): Volume of plasma cleared of drug per unit time.

What the DRUG does to the BODY.

Mechanism of Drug Action: How drugs interact with biological systems to produce effects.
- Receptors: Most drugs act by binding to specific macromolecules (receptors), usually proteins (G-protein coupled, ligand-gated ion channels, enzyme-linked, intracellular).
- Enzymes: Drugs can inhibit or activate enzymes.
- Ion Channels: Drugs can block or open channels.
- Transporters: Drugs can inhibit uptake or efflux transporters.
- Non-receptor mechanisms: Antacids (neutralize acid), osmotic diuretics (osmotic effect).
Agonist: Binds to receptor and activates it, producing a biological response. (Full, partial, inverse).
Antagonist: Binds to receptor but does not activate it. Blocks agonist action. (Competitive, non-competitive, irreversible, physiological, chemical).
Dose-Response Relationship:
- Potency: Amount of drug needed to produce an effect. $EC_{50}$ (concentration for 50% max effect).
- Efficacy: Maximum effect a drug can produce ($E_{max}$).
- Therapeutic Index (TI): Ratio of toxic dose to effective dose. $TI = \frac{TD_{50}}{ED_{50}}$. A measure of drug safety. Higher TI = safer drug.

General Anesthetics: Reversible loss of consciousness and sensation.
- Inhalational: Sevoflurane, Isoflurane, Desflurane, Nitrous Oxide.
  - Mechanism: Enhance GABAergic inhibition, inhibit NMDA receptors.
  - MAC (Minimum Alveolar Concentration): Potency measure. Lower MAC = higher potency.
- Intravenous: Propofol, Thiopental, Ketamine, Etomidate, Midazolam.
  - Propofol: Rapid onset, short duration, antiemetic.
  - Ketamine: Dissociative anesthesia, analgesia, sympathetic stimulation.
Local Anesthetics: Reversible blockade of nerve conduction at site of application.
- Mechanism: Block voltage-gated $\text{Na}^+$ channels, preventing nerve impulse transmission.
- Esters (e.g., Procaine, Cocaine): Metabolized by plasma esterases.
- Amides (e.g., Lidocaine, Bupivacaine): Metabolized by hepatic CYP450.