One-compartment PK with first-order absorption (k_a ≈ 4 h⁻¹). Y-axis is body load in mg/kg, summed across boluses (linear PK assumed). Paraxanthine modeled as a cascade with f ≈ 0.84 of caffeine elimination feeding it. "Effective caffeine" = caffeine + (par equiv) · paraxanthine. Defaults are population averages; individual variation is large (CYP1A2 polymorphism, smoking, oral contraceptives, pregnancy all matter).