**Question:** Adverse effect of phenytoin include all of the following except:

A. Paresthesia
B. Gastrointestinal disturbances
C. Drowsiness
D. Convulsions

**Core Concept:**

Phenytoin is an anti-seizure medication primarily used for the treatment of epilepsy. It works by inhibiting the release of excitatory neurotransmitters, particularly glutamate, which helps in reducing neuronal excitability and preventing seizures. However, like all medications, phenytoin can cause various adverse effects due to its pharmacological actions and interactions with other drugs.

**Why the Correct Answer is Right:**

The correct answer is "D. Convulsions", as phenytoin is primarily used for the treatment of seizures. When the medication is effective, it reduces the incidence of seizures, but if the dose is increased beyond the therapeutic range, it can lead to convulsions or seizures. The other options are:

A. Paresthesia: This is a common adverse effect of phenytoin, which manifests as tingling or numbness, usually on the hands and feet.
B. Gastrointestinal disturbances: Phenytoin can cause nausea, vomiting, and diarrhoea, making this option incorrect as it is an adverse effect rather than a lack of effect.
C. Drowsiness: Phenytoin can induce sedation, which is a known adverse effect.

**Why Each Wrong Option is Incorrect:**

A. Paresthesia: As mentioned above, this is a common adverse effect of phenytoin and not a lack of effect.
B. Gastrointestinal disturbances: As mentioned, this is an adverse effect, not a lack of effect.
C. Drowsiness: This is another adverse effect of phenytoin, which is not related to a lack of effect.

**Clinical Pearl:**

Understanding the adverse effects of medications is crucial for safe and effective prescription and management of patients. In clinical practice, it is essential to monitor patients on phenytoin therapy for side effects like paresthesia, gastrointestinal disturbances, and drowsiness, as these may require dose adjustments or substitution with alternative anti-seizure medications.