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What is hydrochlorothiazide?

Hydrochlorothiazide is a diuretic medication used to treat high blood pressure (hypertension) and other conditions. It is a thiazide diuretic, meaning it helps remove excess water from your body by making you urinate more often.

The medication is available as a tablet or capsule and is usually taken once daily.

What is hydrochlorothiazide used to treat?

Hydrochlorothiazide is mostly used to treat high blood pressure and leg edema (swelling), but it can also be used for other conditions as determined by your doctor.

Taking the drug can help lower your blood pressure and reduce the risk of heart attack, stroke, and kidney problems.

How do you take hydrochlorothiazide?

The hydrochlorothiazide dose your doctor prescribes will be determined by a number of variables, including:

  • The nature and severity of the condition

  • Your age

  • The form of hydrochlorothiazide being prescribed

  • Any other medical conditions you have, such as renal damage

Typically, your doctor will start by prescribing a low dose and gradually increasing it until you find the optimal dosage for you. They’ll finally prescribe the lowest effective dose.

People usually take hydrochlorothiazide once a day.¹

Take it with or without food in the morning. Taking the medication shortly before you go to sleep might cause you to wake up during the night to urinate, so take your dose at least four hours² before heading to bed.

Never take the medication less or more often than your doctor prescribes, and don’t start or stop taking it without medical guidance.

Seeing results

It usually takes about two to four days until you notice hydrochlorothiazide working. For example, you might notice less fluid retention and lower blood pressure readings.

If you don’t see any improvements after this amount of time, your doctor may increase your dose or recommend a different course of treatment.

For the best chance of success, follow your doctor’s instructions and take hydrochlorothiazide exactly as prescribed. If you don’t, it will be less effective and you may be more likely to experience side effects.

Potential side effects of hydrochlorothiazide

Most people don’t experience any serious side effects when taking hydrochlorothiazide, but a small number of people may develop the following:

  • Electrolyte imbalance

  • Low blood pressure

  • Dizziness or lightheadedness

  • Headache

  • Nausea or vomiting

  • Diarrhea

  • Muscle weakness

  • Skin rash

Hydrochlorothiazide may make your skin more sensitive to sunlight, so you should try to avoid prolonged sun exposure, tanning booths, and sunlamps. Wear protective clothing and use sunscreen when outdoors.

Contact your doctor immediately if you develop serious side effects. Side effects can usually be well managed, but in rare cases, they may be more serious and require medical attention.

Speak to your doctor as soon as possible if your side effects continue or worsen.

Long-term use of hydrochlorothiazide

There are some potential risks³ associated with the long-term use of hydrochlorothiazide. These include:

  • Low blood potassium levels (hypokalemia), potentially causing fatigue, muscle weakness, and cramping. For this reason, your doctor might also prescribe potassium pills to be taken daily to help keep your potassium levels in the normal range during hydrochlorothiazide treatment.

  • High blood uric acid levels (hyperuricemia), potentially leading to gout or kidney stones.

  • Low blood sodium levels (hyponatremia), which can cause fatigue, confusion, and muscle cramps.

  • High blood sugar levels (hyperglycemia), potentially resulting in increased thirst and urination, as well as weight gain.

  • High blood calcium levels (hypercalcemia), possibly causing kidney stones or bone loss.

Your doctor may test you regularly for these problems during hydrochlorothiazide treatment. If necessary, they may alter your dose or prescribe a different medication.

Missed doses

If you miss a dose of hydrochlorothiazide, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not take two doses at once to make up for a missed one.


An overdose of hydrochlorothiazide may be fatal. Symptoms of an overdose may include:

  • Drowsiness

  • Confusion

  • Agitation

  • Nausea or vomiting

  • Abnormally slow heartbeat

  • Shortness of breath or difficulty breathing

If you think you or someone else has overdosed on hydrochlorothiazide, seek medical attention immediately.

What to discuss with your doctor before taking hydrochlorothiazide

Before taking hydrochlorothiazide, your doctor should be aware of any factors that might influence how the drug works or make it unsafe for you to use.

Let your doctor know if you are allergic to hydrochlorothiazide or any other medication (especially sulfa drugs). Also tell your doctor about any other medications you are currently taking, both prescription and over-the-counter. Herbal medicines and nutritional supplements may also interact with drugs, so tell your doctor if you take any or plan to start.

Ensure your doctor is aware of your medical history. Some medical conditions, such as diabetes, gout, kidney disease, or lupus, might mean hydrochlorothiazide isn’t a safe choice of medication, or you need close monitoring during treatment.

Finally, be sure to tell your doctor if you are pregnant or breastfeeding.

Stopping hydrochlorothiazide

If you stop taking hydrochlorothiazide suddenly, your blood pressure could increase and raise your risk of complications. You may experience headaches, dizziness, or a fast heartbeat if you stop taking this medication suddenly.

You should only stop taking hydrochlorothiazide if your doctor recommends doing so, and they may taper off the medication slowly to avoid side effects.

Hydrochlorothiazide and pregnancy

Hydrochlorothiazide is listed as a US Food and Drug Administration (FDA) pregnancy category C⁴ drug, meaning it’s unknown whether hydrochlorothiazide is harmful to an unborn baby. Do not take this medication without first talking to your doctor if you are pregnant or could become pregnant during treatment.

Hydrochlorothiazide passes into breast milk and may harm a nursing baby. Do not take this medication without first talking to your doctor if you are breastfeeding.

If you become pregnant while taking hydrochlorothiazide, call your doctor. You should not stop taking hydrochlorothiazide suddenly. Doing so could cause dangerous side effects.

Interactions with other drugs

Tell your doctor about all other medications you are taking, including over-the-counter drugs, vitamins, and herbal supplements. That way, your doctor can check for any possible drug interactions.

If you have diabetes, hydrochlorothiazide may affect your blood sugar levels. Your doctor may need to adjust your diabetes medication if you take this drug.

If you have kidney disease, your doctor may start you on a lower dose of this medication or monitor you more closely for side effects.

Do not take potassium supplements or salt substitutes while taking hydrochlorothiazide, unless your doctor has told you to do so.

Allergy information

Hydrochlorothiazide can cause a severe allergic reaction. Symptoms can include:

  • Trouble breathing

  • Swelling of your throat or tongue

  • Hives

Seek emergency medical help in the event of an allergic reaction to hydrochlorothiazide.

Clinical trial history

A randomized, double-blind clinical trial⁵ assessed the efficacy of canrenone (a diuretic) against hydrochlorothiazide for treating 182 people with hypertension and type 2 diabetes mellitus. The participants were already taking angiotensin receptor blockers (ARBs).

For one month, participants were randomly assigned either a once-daily 50mg dose of canrenone or 12.5mg of hydrochlorothiazide. Trial participants whose blood pressure did not reduce sufficiently were titrated to 100mg of canrenone or 25mg of hydrochlorothiazide once a day for 12 months.

At six and 12 months, the drugs’ effect on blood pressure was assessed among other factors. Both therapies resulted in a comparable drop in blood pressure by the halfway point and by the end of the trial.

Percentage of patients who reached target systolic/diastolic blood pressure target after six months:

The study suggests that canrenone may be slightly more effective in lowering blood pressure than hydrochlorothiazide after six months of treatment.

However, after 12 months, all participants reached their systolic and diastolic blood pressure targets, regardless of which medication they took.

Tips and advice for taking hydrochlorothiazide

Here are some tips to improve your experience and keep you safe when taking hydrochlorothiazide:

  • Take your medication as prescribed by your doctor. Don’t start or stop treatment without their guidance.

  • If your doctor prescribes daily potassium pills to take along with hydrochlorothiazide, do not skip doses.

  • Keep in mind that you will likely need routine blood work while taking hydrochlorothiazide to ensure that your potassium and kidney function levels are staying within normal ranges. Discuss this with your doctor.

  • Drink plenty of water while taking hydrochlorothiazide to prevent dehydration.

  • Taking the medication with food may help reduce nausea.

  • Notify your doctor if you experience any side effects.

  • Tell your doctor if you plan to start taking any other medications, supplements, or herbal medicines while undergoing hydrochlorothiazide treatment.

  • Talk to your doctor if you don’t think the medication is working for you. Your dose might need to be changed.

Remember that for most people, hydrochlorothiazide is a safe and effective medication when used exactly as directed.

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.