What oral vitamin C dosage is recommended for an adult without a history of calcium oxalate kidney stones, glucose‑6‑phosphate dehydrogenase deficiency, or iron‑overload disorders?

Vitamin C Prescription for Adults Without Contraindications

For adults without calcium oxalate kidney stones, G6PD deficiency, or iron-overload disorders, prescribe oral vitamin C at the U.S. Recommended Dietary Allowance (RDA): 90 mg/day for men and 75 mg/day for women, with a maximum safe upper limit of 2,000 mg/day. 1

Standard Dosing Recommendations

The routine daily multivitamin should follow the DRI (Dietary Reference Intake) recommendation for vitamin C, which is 90 mg/day for adult men and 75 mg/day for adult women. 1 This baseline recommendation applies to general health maintenance and prevention of deficiency.

Upper Safety Limits

• The tolerable upper intake level is 2,000 mg/day for adults; exceeding this dose significantly increases the risk of adverse effects, particularly hyperoxaluria and kidney stone formation. 2
• Doses above 500-1,000 mg/day should be avoided in routine supplementation due to the risk of oxalate nephropathy. 3, 4, 5

Clinical Context for Higher Doses

Osteoporosis Management

For patients requiring vitamin C as part of bone health optimization (e.g., those on glucocorticoids or with osteopenia), the routine daily multivitamin containing DRI-level vitamin C (approximately 60-90 mg) is sufficient. 1 There is no evidence supporting megadoses of vitamin C for bone health.

Scurvy Treatment

For documented scurvy requiring parenteral therapy, the FDA-approved dosing is:

• Adults and pediatric patients ≥11 years: 200 mg once daily intravenously for up to 1 week 6
• This indication is restricted to patients for whom oral administration is impossible, insufficient, or contraindicated 6

Critical Safety Considerations

Oxalate Nephropathy Risk

High-dose vitamin C (≥500-1,000 mg/day) can cause secondary hyperoxaluria and acute or chronic oxalate nephropathy, particularly when combined with risk factors such as:

• Dehydration or low fluid intake 3, 5
• Chronic diarrhea or malabsorption 3
• Metabolic acidosis 3
• Pre-existing renal insufficiency 7
• Oxalate-rich diet 3

A 73-year-old patient developed acute kidney injury (creatinine rising from 1.2 to 8.4 mg/dL) after chronic use of 680 mg/day vitamin C, requiring hemodialysis; renal function partially recovered only after discontinuation. 3 Another case documented progressive chronic kidney disease from 3 g/day vitamin C supplementation during COVID-19. 4

Hemodialysis Patients

In hemodialysis patients, even 500 mg/day vitamin C supplementation causes significant hyperoxalemia (mean plasma oxalate 50.4 μmol/L vs. 25.7 μmol/L without supplementation), with a strong correlation between vitamin C and oxalate levels (r=0.755, p<0.01). 7 Restrict vitamin C to 50-100 mg/day in dialysis patients to correct deficiency without inducing hyperoxalemia. 7

Monitoring Recommendations

For patients taking vitamin C supplements >100 mg/day chronically:

• Monitor urinary oxalate-to-creatinine ratio (normal <0.035) every 3-6 months 3
• Assess serum creatinine at baseline and every 3-6 months 3
• Maintain adequate hydration (urine output >2.0-2.5 L/day) 2

If unexplained renal insufficiency develops in a patient on vitamin C supplements, immediately:

• Measure 24-hour urinary oxalate excretion 3
• Discontinue vitamin C supplementation 3, 4
• Consider renal biopsy if hyperoxaluria is confirmed and renal function continues to decline 3

Practical Prescribing Algorithm

1. Assess indication: Is there documented scurvy or specific deficiency requiring treatment?

   • If yes → Treat with 100-200 mg/day orally until resolution 1
   • If no → Proceed to step 2

2. Evaluate baseline risk factors:

   • History of kidney stones? → Avoid vitamin C supplements; obtain calcium and vitamin C from diet only 2
   • Chronic kidney disease (any stage)? → Limit to 50-100 mg/day maximum 7
   • Malabsorption or chronic diarrhea? → Limit to 100 mg/day and monitor closely 3

3. For general supplementation in healthy adults:

   • Prescribe a standard multivitamin containing 60-90 mg vitamin C 1
   • Counsel patient to avoid additional standalone vitamin C supplements 4, 2
   • Emphasize adequate fluid intake (2.5-3.0 L/day) 2

Common Pitfalls to Avoid

• Do not prescribe "megadose" vitamin C (≥1,000 mg/day) for immune support or general wellness; this practice became widespread during COVID-19 but carries significant nephrotoxicity risk. 4
• Do not assume over-the-counter vitamin C is harmless; patients often self-administer 1,000-3,000 mg/day without medical supervision. 4, 5
• Do not overlook vitamin C content in multiple supplements; patients may unknowingly exceed safe limits by combining a multivitamin, immune support formula, and standalone vitamin C. 4
• Do not continue high-dose vitamin C indefinitely; even short-term use (weeks to months) can cause irreversible renal damage. 3, 4