METAHYDRIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for METAHYDRIN (METAHYDRIN).
Metahydrin (trichlormethiazide) is a thiazide diuretic that inhibits the sodium-chloride symporter (NCC) in the distal convoluted tubule of the nephron, reducing sodium and chloride reabsorption and increasing excretion of water, sodium, chloride, and potassium.
| Metabolism | Metahydrin is not extensively metabolized; primarily excreted unchanged by the kidneys. |
| Excretion | Renal: 30% (fecal: 70% as unabsorbed drug, primarily biliary elimination; <1% unchanged in urine) |
| Half-life | 18-30 hours (clinically relevant for once-daily dosing in hypertension; prolonged in renal impairment) |
| Protein binding | 90-95% (bound to albumin and alpha-1-acid glycoprotein) |
| Volume of Distribution | 1.5-2.5 L/kg (indicates extensive tissue distribution, binding to vascular smooth muscle) |
| Bioavailability | Oral: 90-100% (well absorbed, minimal first-pass metabolism) |
| Onset of Action | Oral: 1-2 hours; peak effect at 4-6 hours |
| Duration of Action | 12-24 hours (antihypertensive effect persists for 24 hours, allowing once-daily dosing) |
Oral, 50-100 mg once daily. Maximum 200 mg/day.
| Dosage form | TABLET |
| Renal impairment | GFR 30-<60 mL/min: 50 mg once daily. GFR <30 mL/min: 25 mg once daily or every other day. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: 50 mg once daily. Child-Pugh C: avoid use. |
| Pediatric use | Not recommended for pediatric use; safety not established. |
| Geriatric use | Start at 25 mg once daily; titrate slowly. Monitor renal function. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for METAHYDRIN (METAHYDRIN).
| Breastfeeding | Thiazides are excreted in breast milk in small amounts. M/P ratio not well-defined for metahydrin. May suppress lactation and cause neonatal electrolyte disturbances. Use generally avoided in nursing mothers. |
| Teratogenic Risk | First trimester: Thiazide diuretics are generally avoided due to potential teratogenic effects, including neural tube defects and cardiovascular anomalies. Second and third trimesters: Associated with fetal or neonatal jaundice, thrombocytopenia, electrolyte imbalances, and possibly hypoglycemia. May cause decreased placental perfusion. |
■ FDA Black Box Warning
No FDA black box warning.
| Serious Effects |
["Anuria","Renal failure","Hypersensitivity to trichlormethiazide or other sulfonamide-derived drugs"]
| Precautions | ["Electrolyte disturbances (hypokalemia, hyponatremia, hypochloremic alkalosis)","Hyperuricemia and gout","Hypotension and orthostatic hypotension","Sulfonamide hypersensitivity reactions","Exacerbation of systemic lupus erythematosus","Photosensitivity","Increased risk of non-melanoma skin cancer","Acute angle-closure glaucoma (rare)"] |
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| Fetal Monitoring |
| Monitor maternal blood pressure, serum electrolytes (especially potassium, sodium), uric acid, blood glucose, and renal function. Fetal monitoring includes ultrasound for growth and amniotic fluid volume, as thiazides can cause oligohydramnios. |
| Fertility Effects | Limited data. Thiazides may interfere with placental hormone function but no direct evidence of impaired fertility. Long-term use may affect menstrual regularity due to electrolyte disturbances. |