Hydrochlorothiazide

Key points

• It can lower blood potassium and sodium levels, and thiazides increase the urinary loss of magnesium, which may lead to low magnesium levels.
• As a sulfonamide, it can cause an idiosyncratic reaction leading to acute transient myopia and acute angle-closure glaucoma, with sudden blurred vision or eye pain usually appearing within hours to weeks of starting treatment.
• Hypersensitivity reactions to hydrochlorothiazide can occur in anyone, though the label notes they are more likely in people with a history of allergy or bronchial asthma.

How it works

Hydrochlorothiazide contributes antihypertensive action as a thiazide diuretic. Within Micardis HCT, this is paired with telmisartan, an angiotensin II receptor blocker (ARB), combining two antihypertensive drugs in one tablet.

Safety specific to hydrochlorothiazide

Products that combine hydrochlorothiazide with a medicine that acts directly on the renin-angiotensin system, such as telmisartan in Micardis HCT, carry a boxed warning on fetal toxicity. When pregnancy is detected, Micardis HCT should be discontinued as soon as possible, because drugs that act directly on the renin-angiotensin system can cause injury and death to the developing fetus.

Hydrochlorothiazide itself can cause hypokalaemia and hyponatraemia, and thiazides increase the urinary excretion of magnesium, which may result in hypomagnesaemia. As a sulfonamide, hydrochlorothiazide can also cause an idiosyncratic reaction resulting in acute transient myopia and acute angle-closure glaucoma; symptoms include sudden decreased visual acuity or eye pain, typically within hours to weeks of starting the medicine, and untreated acute angle-closure glaucoma can lead to permanent vision loss. Hypersensitivity reactions to hydrochlorothiazide may occur in anyone but are more likely in people with a history of allergy or bronchial asthma. Thiazide diuretics have also been reported to cause exacerbation or activation of systemic lupus erythematosus, and the antihypertensive effect of hydrochlorothiazide may be enhanced in patients who have had a sympathectomy.

When to seek urgent care

Sudden blurred vision or eye pain after starting a hydrochlorothiazide-containing medicine needs urgent assessment: these can be symptoms of acute transient myopia or acute angle-closure glaucoma, which typically appear within hours to weeks of starting the medicine, and untreated acute angle-closure glaucoma can lead to permanent vision loss.

Further reading

• MICARDIS HCT- telmisartan and hydrochlorothiazide tablet
• Hydrochlorothiazide: MedlinePlus Drug Information
• Micardis 80 mg tablets