Terms in Pharmacology, Part 1: Dose, Side Effects, Contraindications and Allergy
8 minute read
What you will learn
- Define dose, dosage form and therapeutic effect
- Distinguish a side effect from an adverse reaction
- Define contraindication and give examples
- Distinguish sensitivity, hypersensitivity, allergy and idiosyncrasy
- Explain compliance and why it matters
- Use the terms correctly in sentences
Key points
- A dose is the amount of a medicine given at one time. A dosage form is the physical form in which it is given.
- A side effect is an effect other than the one wanted. Some are mild and some are serious.
- An adverse drug reaction is any harmful and unintended response to a medicine at normal doses.
- A contraindication is a condition or situation in which a medicine should not be used because it may cause harm.
- Hypersensitivity and allergy are immune reactions, and can be dangerous, as in anaphylaxis.
- Idiosyncrasy is an unusual reaction, often due to inherited differences.
- Compliance (adherence) is how closely a patient follows the treatment plan.
Dose and dosage form
| Term | Meaning | Example |
|---|---|---|
| Dose | The amount of a medicine given at one time | 500 mg of paracetamol |
| Dosage regimen | The dose, how often, and for how long | 500 mg three times a day for 5 days |
| Dosage form | The physical form of the medicine | Tablet, capsule, syrup, injection, cream |
| Therapeutic effect | The wanted effect that the medicine is given for | Pain relief from paracetamol |
Other dose terms you may meet:
- Minimum effective dose: the smallest dose that gives the wanted effect.
- Maximum (toxic) dose: the highest dose that can be given without harm. Above it, toxicity appears.
- Loading dose: a larger first dose to reach an effective level quickly.
- Maintenance dose: the regular dose that keeps the level steady.
- Lethal dose: a dose that can cause death.
Side effects and adverse reactions
| Term | Meaning |
|---|---|
| Side effect | An effect other than the wanted one, which occurs at normal doses. May be mild, such as drowsiness, or serious |
| Adverse drug reaction (ADR) | Any harmful and unwanted response to a medicine at normal doses |
| Toxic effect | Harm from too much drug, or from a drug that builds up |
Examples:
- Drowsiness with some antihistamines
- Stomach irritation with NSAIDs such as ibuprofen
- Dry cough with some blood pressure medicines
- Nausea with many antibiotics
Patients should be told the common side effects and what to do about them. Serious reactions must be reported to the pharmacist or doctor at once. Reporting adverse reactions is part of pharmacovigilance.
Contraindication
A contraindication is a condition or situation in which a medicine should not be used, because it may cause harm.
| Medicine | Contraindication (example) |
|---|---|
| Aspirin and NSAIDs | Active stomach ulcer |
| Penicillins | Known penicillin allergy |
| Many medicines | Pregnancy, when a drug may harm the baby |
| Metformin | Severe kidney disease |
| Medicines that cause drowsiness | Driving, for the patient's safety (a caution) |
- An absolute contraindication means the medicine must never be used.
- A relative contraindication means the medicine may be used with great care, if the benefit outweighs the risk, and the doctor decides.
A precaution or caution is a lesser warning: the medicine may be used but needs care.
Sensitivity, hypersensitivity, allergy and idiosyncrasy
| Term | Meaning |
|---|---|
| Sensitivity | How responsive a person is to a drug. Some people respond to small doses |
| Hypersensitivity | An exaggerated reaction to a drug, often involving the immune system |
| Allergy | A hypersensitivity reaction by the immune system, such as a rash, itching, swelling or difficulty breathing |
| Anaphylaxis | A severe, life-threatening allergic reaction needing emergency treatment |
| Idiosyncrasy | An unusual reaction to a drug that is not expected from its action, often due to inherited differences |
Allergy warning signs
- Skin rash, hives or itching
- Swelling of the face, lips or tongue
- Wheezing or difficulty breathing
- Dizziness or collapse
If these occur, the patient needs urgent medical help. Always ask about allergies before dispensing, and record them. Penicillin allergy is a common example.
A side effect is not the same as an allergy. For example, nausea from an antibiotic is a side effect. A rash with swelling suggests an allergy.
Compliance (adherence)
Compliance, also called adherence, is how closely a patient follows the treatment plan: taking the right dose, at the right time, for the right length of time.
Poor compliance causes:
- treatment failure and relapse;
- resistance, for example to antibiotics and antimalarials;
- waste of money and medicines.
Reasons for poor compliance include forgetting, side effects, cost, complex regimens, not understanding, and feeling better.
How you can help
- Give clear, simple instructions and labels.
- Explain why the course must be finished.
- Use reminders, pill boxes and simple schedules.
- Ask the patient to repeat the instructions.
- Refer concerns about side effects or cost to the pharmacist.
Exam focus
- Define each term in one sentence and give an example.
- State the difference between a side effect and an allergy.
- Give an example of a contraindication.
- Common mistake: calling any side effect an allergy.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.A dose is:
2.A contraindication is:
3.A patient develops a rash and swelling of the face after a dose of penicillin. This is most likely:
4.Drowsiness after taking chlorpheniramine, an antihistamine, is an example of:
5.A patient stops an antibiotic after 2 days because they feel better. This is a problem of: