Overview
Hydrochlorothiazide is a thiazide diuretic (water pill) widely used to treat hypertension (high blood pressure) and oedema (fluid retention). By helping the kidneys remove excess water and salt from the body, it effectively lowers blood pressure and reduces swelling.

Mechanism of Action
It acts on the distal convoluted tubule in the kidneys to inhibit the sodium-chloride symporter. This action decreases the reabsorption of sodium and chloride, leading to increased excretion of water, sodium, chloride, and potassium in the urine. The reduction in blood volume results in lowered blood pressure.

Dosage and Administration
For hypertension, the usual starting dose for adults is 12.5 mg to 25 mg once daily, which may be increased to 50 mg daily. For oedema, the dose ranges from 25 mg to 100 mg daily in single or divided doses. It is usually taken in the morning to prevent frequent urination during the night.

Side Effects
Common side effects include frequent urination, dizziness, headache, and electrolyte imbalances such as hypokalaemia (low potassium) and hyponatraemia (low sodium). It may also elevate blood sugar and uric acid levels, potentially triggering gout attacks in susceptible individuals.

Contraindications
It is contraindicated in patients with anuria (inability to produce urine) or a known hypersensitivity to thiazides or sulfonamide-derived drugs. It should be used with caution in patients with severe renal or hepatic impairment.

Pregnancy and Lactation
Use during pregnancy is generally not recommended unless strictly necessary, as it may decrease placental perfusion and cause foetal harm. It is excreted in breast milk and may inhibit lactation, so alternative treatments are usually preferred for nursing mothers.