Intolerance to a drug occurs at what dose in an individual –
**Core Concept**
Intolerance to a drug occurs when a patient experiences an adverse effect at a dose that is lower than the usual therapeutic dose. This phenomenon is often seen in patients with genetic variations, concurrent illnesses, or interactions with other medications.
**Why the Correct Answer is Right**
Intolerance to a drug is usually due to an abnormal pharmacokinetic or pharmacodynamic response. Patients with intolerance may have altered enzyme activity, reduced renal clearance, or increased receptor sensitivity, leading to adverse effects at lower doses. For example, patients with reduced CYP2D6 enzyme activity may experience increased sedation with standard doses of codeine.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the dose at which intolerance occurs. Intolerance can occur at any dose, but it is typically seen at lower doses than the usual therapeutic dose.
**Option B:** This option is incorrect because it implies that intolerance only occurs in patients with a history of allergy or hypersensitivity. While some patients with allergies may experience intolerance, this is not the only cause.
**Clinical Pearl / High-Yield Fact**
Remember that intolerance to a drug can occur at any dose, but it is often seen in patients with genetic variations or concurrent illnesses. Always consider a patient's individual factors when prescribing medications.
**Correct Answer:** D.