Pharmacology
Pharmacokinetics (ADME)
How the body processes drugs over time
Pharmacokinetics is the study of how the body processes a drug, categorized by the four ADME stages: Absorption, Distribution, Metabolism, and Excretion. A drug's duration in the system varies wildly—the half-life of aspirin is only 15–20 minutes, whereas fluoxetine can remain for up to 3 days. Understanding the "therapeutic window" is vital, as plasma concentration must remain high enough to be effective but low enough to avoid toxicity.
- ADMEAbsorption, Distribution, Metabolism, Excretion
- Half-life examplesAspirin (20m) vs. Fluoxetine (3d)
- Therapeutic WindowRange between efficacy and toxicity
- First-pass effectLiver metabolism before reaching blood
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How it works
Pharmacokinetics describes what the body does to the drug. Absorption is how the drug enters the blood (e.g., through the gut). Distribution is where it goes (muscles, fat, brain). Metabolism is how the body breaks it down, primarily in the liver. Excretion is how it leaves, usually via the kidneys. These factors determine the plasma concentration-time curve, which doctors use to calculate the correct dose and frequency.
The Therapeutic Window
Every drug has a Minimum Effective Concentration (MEC) and a Minimum Toxic Concentration (MTC). The space between them is the therapeutic window. If the dose is too low, the drug is a 'sub-therapeutic' failure. If it is too high, it becomes poisonous. Maintaining a steady state within this window often requires 'loading doses' followed by 'maintenance doses' to account for the drug's half-life.
Common pitfalls
- Ignoring First-Pass Metabolism: Some drugs are 90% destroyed by the liver before they ever reach the heart if taken orally.
- Assuming uniform distribution: Fat-soluble drugs can 'hide' in adipose tissue, extending their duration unexpectedly.
- Drug-drug interactions: One drug can inhibit the liver enzymes needed to metabolize another, causing toxic buildup.
| Feature | Pharmacokinetics | Pharmacodynamics |
|---|---|---|
| Core Question | What does the body do to the drug? | What does the drug do to the body? |
| Focus | ADME (Flow and timing) | Receptors and signaling |
| Key Metric | Half-life, Clearance, Volume of distribution | Potency, Efficacy, EC50 |
| Visual Tool | Concentration-Time Curve | Dose-Response Curve |
Frequently asked questions
What is a drug's half-life?
The time it takes for the concentration of a drug in the blood to decrease by exactly 50%. It determines how often you need to take a pill to stay in the therapeutic window.
Why do some drugs say 'take with food'?
Food can change the <strong>Absorption</strong> phase by slowing down stomach emptying or helping fat-soluble drugs dissolve, which alters the drug's peak concentration (Cmax).
What is the first-pass effect?
When a drug is swallowed, it goes to the liver first via the portal vein. The liver may metabolize a huge portion of it before it reaches the rest of the body. This is why some drugs must be given by injection or under the tongue.
What happens in the liver during Metabolism?
Enzymes (like the Cytochrome P450 family) transform the drug into 'metabolites' that are usually more water-soluble and easier for the kidneys to excrete.
Why do elderly people often need lower doses?
As we age, liver metabolism and kidney excretion slow down, meaning drugs stay in the body longer and can reach toxic levels more easily.