Pharmacokinetics: 500mg dose peaks at ~45 μg/mL after 1.2h, with 4h half-life
A 500mg oral dose with 80% bioavailability, absorption rate of 1.5/h, and a 4-hour half-life reaches peak plasma concentration of approximately 45 μg/mL at 1.2 hours post-dose. With 8-hour dosing intervals, steady state is reached after about 5 half-lives (20 hours), with trough concentrations remaining above the therapeutic threshold.
What Happens After You Take a Pill?
When you swallow a medication, a remarkable journey begins. The drug dissolves in your stomach, crosses the intestinal wall into the bloodstream, survives first-pass metabolism in the liver, distributes throughout the body to reach its target, and is eventually eliminated. Pharmacokinetics — literally 'drug movement' — models this entire process mathematically, enabling precise dosing that keeps drug levels in the therapeutic sweet spot.
The ADME Framework
Pharmacokinetics is organized around four processes: Absorption (how the drug enters the bloodstream — rate constant ka and bioavailability F), Distribution (how it spreads to tissues — volume of distribution Vd), Metabolism (how the body chemically transforms it — primarily hepatic), and Elimination (how it is removed — clearance CL and half-life t½). Together, these determine the plasma concentration-time curve that this simulation visualizes.
The Therapeutic Window
The most critical concept in clinical pharmacokinetics is the therapeutic window — the concentration range between the minimum effective concentration (MEC) and the toxic threshold. Too low and the drug doesn't work; too high and side effects appear. The simulation shows this as the green band. Watch how changing the dose and dosing interval affects whether plasma levels stay within this critical range.
Multi-Dose Accumulation
In clinical practice, drugs are given repeatedly. Each new dose adds to the residual drug from previous doses, causing plasma levels to gradually rise until reaching steady state — where the amount absorbed per interval exactly equals the amount eliminated. This takes approximately 4-5 half-lives. The sawtooth pattern in the simulation shows this accumulation beautifully: each peak and trough rises until the curve stabilizes.
FAQ
What is pharmacokinetics?
Pharmacokinetics (PK) is the study of how the body processes drugs over time — encompassing Absorption (drug entering the bloodstream), Distribution (spreading to tissues), Metabolism (chemical transformation, primarily in the liver), and Elimination (removal from the body). It answers the fundamental question: 'What does the body do to the drug?'
What is a therapeutic window?
The therapeutic window is the range of drug concentrations in the blood between the minimum effective concentration (MEC) and the minimum toxic concentration (MTC). The goal of dosing regimens is to keep plasma levels within this window — high enough to be effective but low enough to avoid toxicity. Drugs with narrow therapeutic windows (like warfarin or lithium) require careful monitoring.
What determines drug half-life?
Half-life (t½) is the time required for plasma concentration to decrease by 50%. It depends on the volume of distribution (Vd) and clearance (CL): t½ = 0.693 × Vd / CL. A large Vd (drug distributed widely in tissues) or low CL (slow elimination) both increase half-life. It takes approximately 4-5 half-lives to reach steady state with repeated dosing.
Why do drugs accumulate with repeated dosing?
When a new dose is given before the previous dose is fully eliminated, drug levels build up in the body. With each dose, the trough concentration rises until the amount eliminated per interval equals the amount absorbed — this is steady state. The accumulation factor is approximately 1/(1 - exp(-ke·τ)), where ke is the elimination rate constant and τ is the dosing interval.