Pharmacology: free practice, theory and problems

1. Pharmacokinetics

What is it about?

Pharmacokinetics is what the body does to the drug: absorption, distribution, metabolism and excretion (ADME).

Concepts and formulas

• Bioavailability: the fraction of the dose that reaches the blood unchanged. Intravenously it is 100%.
• First-pass metabolism: drugs taken by mouth pass the liver before reaching the rest of the body, and some of the drug is broken down there.
• The liver metabolises most drugs. The kidneys excrete many of them.
• Half-life ($t_{1/2}$): the time it takes for the concentration in the blood to halve.
• Amount left after time $t$: $D \cdot (1/2)^{t/t_{1/2}}$.
• After 4–5 half-lives the drug is almost completely eliminated, and with regular doses a stable concentration (steady state) has been reached.
• Older people often have reduced kidney function, so drugs excreted by the kidneys can accumulate.

Common mistakes

• Thinking half-life means half the dose is gone after one half-life and the rest after one more.
• Forgetting that reduced kidney or liver function changes dosing.

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2. Pharmacodynamics and adverse effects

What is it about?

Pharmacodynamics is what the drug does to the body: where it acts, how, and how strongly.

Concepts

• Receptor: a protein the drug binds to.
• Agonist: activates the receptor (mimics the body's own substance). Antagonist: blocks the receptor.
• Therapeutic window: the range between the dose that works and the dose that causes toxic effects. Drugs with a narrow window (for example warfarin, digoxin and lithium) need close monitoring, often with blood tests.
• Adverse effect (side effect): an unwanted effect at a normal dose. Allergic reaction: an immune reaction, at worst anaphylaxis.
• Interaction: one drug changes the effect of another.
• Tolerance: the same dose has less effect over time (common with opioids).

Common mistakes

• Calling every unwanted reaction an "allergy". Most are side effects.

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3. Important drug classes

What is it about?

You meet some drug classes on almost every shift. You must know the main effect, the most important side effects and what to observe.

Classes

• Paracetamol: pain relief and fever reduction. The usual maximum daily dose for adults is 4 g, lower with low weight, liver disease or old age. Overdose damages the liver.
• NSAIDs (for example ibuprofen): pain relief and anti-inflammatory. Can cause stomach ulcers, bleeding and kidney damage.
• Opioids (for example morphine and oxycodone): strong pain relief. Observe respiratory rate and consciousness (respiratory depression), and prevent constipation. Antidote: naloxone.
• Anticoagulants: warfarin is monitored with INR, and the antidote is vitamin K. Low-molecular-weight heparin is often given as an injection under the skin. Observe for bleeding.
• Diuretics: furosemide (loop diuretic) causes a lot of urination and can cause low potassium.
• Beta blockers: lower heart rate and blood pressure. Caution in asthma.
• Insulin: lowers blood glucose. Watch for signs of hypoglycaemia: sweating, trembling, restlessness, confusion.

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4. Medication management

What is it about?

Errors in medication management are among the most common causes of patient harm. Fixed routines stop errors from reaching the patient.

Concepts

• "The five rights": right patient, right drug, right dose, right route and right time. Many add right documentation.
• Double-check: two people check independently of each other, especially for high-risk drugs and calculations.
• Routes: p.o. (by mouth), i.v. (into a vein), s.c. (under the skin), i.m. (into a muscle).
• Medication reconciliation: making sure the medication list is correct when the patient is admitted and discharged.
• Incidents must be reported. The goal is learning, not finding someone to blame.

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