Serious side effects to HCTZ are uncommon but can occur. They include:

  • low levels of potassium
  • glaucoma
  • gout
  • allergic reactions

Low Levels of Potassium

About 1 in 10 people taking HCTZ develop low levels of potassium at some time. Potassium is a mineral that your body needs to work properly. It is key for the function of nerve and muscle cells, especially the heart muscles. Low levels can cause symptoms like weakness, fatigue, muscle cramps, thirst, nausea, and vomiting. Very low levels can lead to confusion, seizures, and an irregular heartbeat. This can be life-threatening.

Within 2 to 4 weeks of starting on HCTZ, your doctor will check your electrolyte levels. This includes potassium. This blood test may also be done when a person develops excessive vomiting or another risk for low potassium. To prevent low levels, you should eat lots of potassium-rich foods. These include vegetables, fruit, salmon, and peanut butter. Your doctor may also prescribe a potassium supplement pill.


Glaucoma is a very rare side effect of HCTZ. Only a few cases have been reported in medical journals. It starts suddenly within hours to weeks of starting HCTZ. Common symptoms are headache, eye pain, and vision problems. They are due to increased pressure within the eye. If it is untreated, lasting blindness can occur.

This rare side effect is not more common in people who have previously had glaucoma due to other causes. They do not need to avoid taking HCTZ.


Gout is another uncommon side effect of HCTZ. It occurs when urate crystals collect in a joint. Some people taking HCTZ develop excess uric acid in the blood. This can lead to these crystals. Gout symptoms usually come on suddenly. They include severe pain, redness, and swelling of the affected joint. People who have had gout before, have a family history of gout, or who have chronic kidney failure are more likely to have gout when taking HCTZ.

Allergic Reaction

All medicines can cause an allergic reaction in some people. Some types of HCTZ contain sulfites. People allergic to sulfites have a higher chance of an allergic reaction if they take an HCTZ that contains sulfite. Signs of a serious allergic reaction include difficulty breathing and swelling of the face, lips, tongue, or throat.