How much vit c can we give to pt ? 500mg or 1500mg
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Vitamin C (Ascorbic Acid) Dosing
The answer to "500 mg or 1500 mg" depends entirely on what you're treating. Here's the full picture:
Standard / Normal Requirements (RDA)
| Population | RDA |
|---|---|
| Adult men | 90 mg/day |
| Adult women | 75 mg/day |
| Smokers | +35 mg/day above RDA |
| Pregnancy / lactation | Higher requirements |
Therapeutic Dosing by Indication
| Indication | Dose |
|---|---|
| Scurvy (treatment) | 200 mg/day - symptoms improve within several days |
| Upper respiratory tract infections | 0.2 g up to several grams/day - may slightly reduce duration/symptoms |
| CRPS prevention (wrist fractures) | 500 mg/day x 50 days (per orthopedic trials) |
| Ocular alkali burns | 1-2 g four times daily (not in renal disease) |
| Burn resuscitation (high-dose protocol) | High-dose IV ascorbic acid used adjunctively |
| Sepsis/ICU | Vitamin C + thiamine + hydrocortisone combo studied (evidence still evolving) |
| IV pharmacologic (cancer) | Up to 1 g/kg body weight IV - only parenteral, not oral |
For CRPS prevention, 500 mg/day is the well-studied dose from the Zollinger et al. RCT, referenced in Rockwood & Green's Fractures in Adults (2025).
Absorption Pharmacokinetics - Why High Doses Don't Always Help More
From Harrison's (2025):
- <100 mg in a single dose = almost completely absorbed
- >1 g in a single dose = only ≤50% absorbed
- At higher intakes, enhanced degradation + fecal/urinary excretion occur
So giving 1500 mg in a single oral dose is largely wasteful - only half or less is actually absorbed. Splitting doses (e.g., 500 mg TID = 1500 mg/day) is more rational than a single large dose.
Toxicity / Upper Limits
| Threshold | Risk |
|---|---|
| >2 g in a single dose | Abdominal pain, diarrhea, nausea |
| Chronic high-dose | Theoretical increased kidney stone risk (oxalate) |
| 2 g/day in stone formers | Raises 24-hr urinary oxalate (29-35 mg → 31-41 mg) |
The Tolerable Upper Intake Level (UL) is 2 g/day for adults. Doses beyond this are not recommended for routine supplementation.
Caution in:
- Patients with renal insufficiency or history of oxalate kidney stones - avoid high doses
- G6PD deficiency - high-dose IV ascorbic acid can cause acute hemolysis
Summary Answer
| Dose | Appropriate for... |
|---|---|
| 500 mg/day | CRPS prevention post-fracture; general supplementation in deficiency states |
| 1000-1500 mg/day (divided doses) | Upper limit of reasonable oral supplementation; upper respiratory infections |
| >2000 mg/day | Generally not recommended orally; risk of GI side effects and oxalate issues |
| Pharmacologic IV (up to 1 g/kg) | Only for specific oncology protocols - not routine |
Bottom line: For most patients, 500 mg/day is the practical therapeutic dose that balances efficacy with safety. 1500 mg/day divided (e.g., 500 mg TID) is acceptable short-term but offers limited additional benefit over 500 mg due to saturation kinetics. Single-dose 1500 mg is not recommended.
Sources: Harrison's Principles of Internal Medicine 22E (2025); Tietz Textbook of Laboratory Medicine 7th Ed; Brenner & Rector's The Kidney; Rockwood & Green's Fractures in Adults 2025; Kanski's Clinical Ophthalmology 10th Ed
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