Side Effects of Hydrochlorothiazide (HCTZ)
Hydrochlorothiazide causes electrolyte disturbances (hypokalemia, hyponatremia, hypomagnesemia), metabolic abnormalities (hyperglycemia, hyperuricemia, hyperlipidemia), volume depletion with hypotension, and carries an increased risk of non-melanoma skin cancer with chronic use. 1
Electrolyte and Metabolic Disturbances
Common Electrolyte Abnormalities
- Hypokalemia is the most clinically significant electrolyte disturbance, particularly when HCTZ is combined with loop diuretics or when baseline potassium is ≤4.3 mmol/L 2
- Hyponatremia occurs dose-dependently and is more common in elderly patients 3, 4
- Hypomagnesemia frequently accompanies hypokalemia and can exacerbate cardiac arrhythmias 5
- These electrolyte depletions predispose patients to serious cardiac arrhythmias, especially in the presence of digitalis therapy 5
Metabolic Effects
- Hyperglycemia and glucose intolerance develop with HCTZ use, with diabetes incidence reaching 11.8% after 4 years of chlorthalidone therapy in ALLHAT 6
- In type 2 diabetics, HCTZ significantly increases fasting glucose (SMD = 0.27) and HbA1c (SMD = 1.09) 7
- Hyperuricemia is common, though clinical gout is uncommon with doses ≤50 mg/day of HCTZ 6
- Dyslipidemia occurs, with HDL-cholesterol decreasing (SMD = -0.44) and hypertriglyceridemia developing 8, 7
Cardiovascular and Hemodynamic Effects
- Hypotension, including orthostatic hypotension, may be aggravated by alcohol, barbiturates, narcotics, or other antihypertensive drugs 1
- Volume depletion can impair renal function and exercise tolerance 5
- Falls risk increases in elderly patients due to hypotension and dizziness 4
Renal Effects
- Renal dysfunction and azotemia can develop, particularly with excessive diuresis 5, 1
- HCTZ causes greater renal injury than equivalent hypokalemia from dietary potassium restriction alone, including glomerular ischemia, medullary injury, and cortical oxidative stress 8
- Interstitial nephritis and renal failure have been reported 1
- In patients with creatinine >150 μmol/L or GFR <30 mL/min, thiazides should not be used except synergistically with loop diuretics 3
Serious Hypersensitivity Reactions
- Anaphylactic reactions can occur, particularly in patients with sulfonamide or penicillin allergies 1, 9
- Severe allergic reactions mimicking septic shock have been documented 9
- Exfoliative dermatitis including Stevens-Johnson syndrome and toxic epidermal necrolysis 1, 10
- Necrotizing angiitis (vasculitis and cutaneous vasculitis) 1
- Respiratory distress including pneumonitis and pulmonary edema 1
Ophthalmologic Emergency
- Acute angle-closure glaucoma is an idiosyncratic reaction that can occur within hours to weeks of initiation 1
- Symptoms include acute onset of decreased visual acuity or ocular pain 1
- Untreated cases can lead to permanent vision loss; discontinue HCTZ immediately 1
Dermatologic Effects
- Photosensitivity reactions are common 1
- Non-melanoma skin cancer risk is significantly increased, predominantly squamous cell carcinoma in white patients taking large cumulative doses (≥50,000 mg) 1
- The increased risk is approximately 1 additional SCC case per 6,700 white patients per year at cumulative doses ≥50,000 mg 1
Hematologic Effects
- Aplastic anemia, agranulocytosis, leukopenia, hemolytic anemia, and thrombocytopenia have been reported 1
Gastrointestinal Effects
- Pancreatitis (rare but serious) 1
- Nausea, vomiting, diarrhea, constipation, gastric irritation, cramping 1
- Intrahepatic cholestatic jaundice 1
Neurologic Effects
Other Effects
- Sexual dysfunction including impotence, with higher incidence at higher doses 6, 1
- Muscle spasm and weakness 1
- Endothelium-dependent vasorelaxation is impaired even with mild hypokalemia 8
Dose-Related Considerations
The adverse effects of HCTZ are dose-related 1. Doses of 12.5 mg daily have adverse events comparable to placebo 1. Higher doses (>50 mg HCTZ or >25 mg chlorthalidone) add little antihypertensive efficacy but significantly increase adverse effects, particularly hypokalemia 6.