RIOMET
Clinical safety rating: caution
Comprehensive clinical and safety monograph for RIOMET (RIOMET).
Biguanide that decreases hepatic glucose production, decreases intestinal absorption of glucose, and improves insulin sensitivity by increasing peripheral glucose uptake and utilization.
| Metabolism | Not metabolized; excreted unchanged in urine. |
| Excretion | Renal (90% unchanged via glomerular filtration and tubular secretion); fecal (10%) |
| Half-life | Terminal elimination half-life: 6.2 hours (range 4–12 hours); clinical context: 4–5 half-lives to steady state (approx 24–36 hours); prolonged in renal impairment (e.g., creatinine clearance <60 mL/min contraindicated) |
| Protein binding | Negligible (<5%) bound to plasma proteins (albumin) |
| Volume of Distribution | 654 ± 358 L (single dose), equivalent to 9.3 L/kg (assuming 70 kg); indicates extensive tissue distribution (concentrates in erythrocytes, liver, kidney; does not bind to plasma proteins) |
| Bioavailability | Oral: 50–60% (absolute); food decreases peak concentration by 40% but extends time to peak; extent of absorption unchanged with food |
| Onset of Action | Oral: 2–4 hours to peak plasma concentration; clinical effect (glucose lowering) begins within 48 hours, maximal by 2 weeks |
| Duration of Action | Duration of glucose-lowering effect: 24 hours (once-daily dosing); clinical notes: twice-daily dosing may prolong duration in some patients; sustained release formulations provide extended duration up to 24 hours |
Oral, 500 mg twice daily or 850 mg once daily, increased gradually to 2000 mg daily in divided doses.
| Dosage form | SOLUTION |
| Renal impairment | Contraindicated if eGFR <30 mL/min/1.73m2; if eGFR 30-45, do not initiate; reduce dose by 50% if eGFR 45-60. |
| Liver impairment | Avoid use in patients with Child-Pugh Class B or C hepatic impairment; no adjustment for Class A. |
| Pediatric use | Initial: 500 mg orally twice daily for children ≥10 years; maximum 2000 mg daily. |
| Geriatric use | Start at lower doses (e.g., 500 mg once daily) due to risk of lactic acidosis; monitor renal function closely. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for RIOMET (RIOMET).
| Breastfeeding | Metformin is excreted into human breast milk. The milk-to-plasma (M/P) ratio is approximately 0.35. Infant exposure is estimated to be less than 1% of the weight-adjusted maternal dose. No adverse effects have been reported in breastfed infants. However, caution is advised, especially in neonates with impaired renal function. Monitor infant for signs of hypoglycemia or gastrointestinal disturbance. |
| Teratogenic Risk | Metformin (Riomet) is classified as FDA Pregnancy Category B. Available data from controlled studies in pregnant women do not show an increased risk of major birth defects. First trimester: No evidence of teratogenicity. Second and third trimesters: No fetal adverse effects observed; placental transfer occurs but at low levels. Risk of neonatal hypoglycemia is low. Use is sometimes continued in GDM if glycemic control is not achieved with insulin. |
■ FDA Black Box Warning
Lactic acidosis: A rare but serious metabolic complication that can occur due to metformin accumulation; risk increases with renal impairment, sepsis, dehydration, acute heart failure, hepatic impairment, and excessive alcohol intake.
| Common Effects | Hypoglycemia low blood glucose level Headache Nausea Diarrhea Flatulence |
| Serious Effects |
["Severe renal impairment (eGFR <30 mL/min/1.73 m²)","Acute or chronic metabolic acidosis (including diabetic ketoacidosis)","Hypersensitivity to metformin","Acute conditions that may alter renal function (e.g., dehydration, sepsis, shock)","Hepatic impairment"]
| Precautions | ["Lactic acidosis risk","Renal impairment (assess renal function before initiation and periodically)","Hypoglycemia risk when used with other antidiabetic agents","Vitamin B12 deficiency with long-term use","Acute heart failure","Hepatic impairment","Discontinue temporarily for IV contrast or surgery","Hypoxic states (e.g., respiratory failure, myocardial infarction)"] |
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| Fetal Monitoring | Maternal: Assess renal function (serum creatinine) prior to and during therapy, especially if at risk for lactic acidosis. Monitor blood glucose levels. Fetal: Standard antenatal monitoring including ultrasound for growth and well-being. In GDM, monitor for macrosomia, polyhydramnios. Neonatal: Observe for hypoglycemia if metformin continued to delivery. |
| Fertility Effects | Metformin may improve fertility in women with polycystic ovary syndrome (PCOS) by reducing hyperinsulinemia and restoring ovulation. No adverse effects on male or female fertility have been reported. It is not considered a fertility drug but can facilitate ovulation in anovulatory women with PCOS. |