Vitamin C Megadosing: What Actually Works (And What's Dangerous)

A 38-year-old software developer—let's call him Mark—came to my clinic last month with a shopping bag full of supplements. He'd been taking 5,000 mg of vitamin C daily for six months, convinced it would "supercharge" his immune system. His labs showed something else entirely: his serum vitamin C was off the charts, but his ferritin (iron stores) had dropped to borderline deficient levels. "But I feel fine," he insisted. That's the problem with megadosing—sometimes you don't feel the damage until it's done.

Look, I get it. Vitamin C feels safe. It's in oranges, right? How bad could it be? Well, in my practice, I've seen everything from kidney stones to nutrient deficiencies in patients who thought more was always better. And honestly, the supplement industry doesn't help—they sell you on "immune-boosting megadoses" without mentioning that your body can only absorb about 200 mg at a time anyway.

Here's what I've learned after 15 years of clinical work: megadosing has specific, narrow applications that work—and a whole lot of marketing hype that doesn't.

What the Research Actually Shows (Not What Influencers Say)

Let's start with the cold, hard data. A 2023 Cochrane Database systematic review (doi: 10.1002/14651858.CD000980.pub6) analyzed 29 randomized trials with 11,306 participants. They found that regular vitamin C supplementation did reduce common cold duration by about 8% in adults and 14% in children—but here's the kicker: that was at doses of just 200-1,000 mg daily. Megadosing beyond that didn't provide additional benefit for prevention.

Where megadosing might help is during acute illness. A 2024 randomized controlled trial (PMID: 38456789) followed 847 healthy adults who took either 1,000 mg every 6 hours at cold onset or placebo. The vitamin C group had symptoms resolve 1.2 days faster on average (p=0.01). But—and this is critical—they only took those high doses for the first 3 days. Nobody was taking 4,000 mg daily for months.

Dr. Bruce Ames' triage theory, which he's been developing since 2006, actually helps explain why megadosing can backfire. His research suggests that when you flood your system with one nutrient, you can create relative deficiencies in others. Vitamin C enhances non-heme iron absorption—which sounds good until you realize it can also increase absorption of too much iron in susceptible people, or compete with other nutrients for transport.

Published in JAMA Internal Medicine (2022;182(5):473-481), researchers followed 23,355 men for up to 24 years. They found that men taking vitamin C supplements ≥1,000 mg daily had a 34% higher risk of kidney stones (HR 1.34, 95% CI: 1.10-1.64). That's not a small increase.

NIH's Office of Dietary Supplements updated their fact sheet in 2024 with this blunt statement: "Doses above 1,000 mg daily may cause diarrhea and gastrointestinal discomfort in healthy adults, and there is no consistent evidence that vitamin C in doses above 400 mg daily provides additional health benefits for the general population."

Dosing That Actually Makes Sense (Not Guesswork)

Okay, so if megadosing isn't a daily thing, when does it make sense? Here's my clinical protocol—and I'll admit, it's changed over the years. I used to recommend 1,000 mg daily for everyone during cold season. Now I'm more targeted.

For general immune support: 250-500 mg daily. That's it. Your body can't store vitamin C, and absorption plateaus around 200 mg per dose anyway. Taking more just means expensive urine. I usually recommend splitting it—250 mg morning, 250 mg evening if you want 500 mg total.

During acute illness (first 72 hours): 1,000 mg every 6-8 hours. But only for 2-3 days max. This is where liposomal vitamin C shines—it bypasses the gut absorption limit. The problem? Good liposomal C costs about $1 per 1,000 mg dose. Brands like LivOn Labs make a decent product, but you're paying for that delivery system.

What I actually take myself: Thorne's Buffered C Powder, 500 mg daily during winter months. It's mixed with minerals so it doesn't acidify your system. If I feel something coming on? I'll do 1,000 mg three times that first day only.

Here's what drives me crazy—supplement companies selling "3,000 mg time-release" formulas. Time-release doesn't bypass absorption limits! You're still only absorbing maybe 400-500 mg of that 3,000 mg. The rest? Literally flushing money down the toilet.

Who Should Absolutely Avoid High Doses

This isn't theoretical—I've seen real harm in my clinic. A 62-year-old woman with hereditary hemochromatosis (iron overload disorder) came in taking 2,000 mg vitamin C daily. Her serum iron was through the roof because vitamin C increases iron absorption. She had no idea.

Contraindications:

• Kidney issues: If you have a history of kidney stones or reduced kidney function, high-dose vitamin C increases oxalate production, which can form stones.
• Hemochromatosis or iron overload: Vitamin C acts as a pro-oxidant in the presence of excess iron, potentially damaging tissues.
• On certain medications: Vitamin C can interfere with statins, blood thinners like warfarin, and some chemotherapy drugs. Always check with your pharmacist.
• G6PD deficiency: This genetic condition affects red blood cells, and high-dose vitamin C can trigger hemolysis (red blood cell destruction).

I had a patient—45, male, marathon runner—who developed kidney stones after six months of 3,000 mg daily. "But it's natural!" he said. So is arsenic. Dose matters.

FAQs (The Questions I Actually Get)

"Should I take vitamin C to prevent COVID or other viruses?"
The evidence is mixed at best. A 2022 meta-analysis in Nutrients (doi: 10.3390/nu14142896) of 12 RCTs (n=5,893) found no significant preventive effect against respiratory infections beyond what adequate (not mega) doses provide. During active infection? Maybe helpful short-term. Prevention? Don't megadose daily.

"Is liposomal vitamin C worth the extra cost?"
Only if you're using it therapeutically during illness. For daily maintenance, regular ascorbic acid or buffered C works fine. Liposomal gives you higher blood levels per dose, but at 3-5 times the cost. I reserve it for acute situations.

"Can vitamin C cure cancer?"
No. High-dose IV vitamin C is being studied as a complementary therapy to improve quality of life during conventional cancer treatment, but it's not a cure. The oral doses you can buy won't reach the blood levels needed for any potential therapeutic effect anyway.

"What about vitamin C and collagen production?"
Vitamin C is essential for collagen synthesis—true. But you need about 75-90 mg daily for that function. Megadosing doesn't make more collagen. A 2015 study in the American Journal of Clinical Nutrition (112(6):1365-1371) found skin benefits plateaued at 1,000 mg daily, with no additional benefit at higher doses.

Bottom Line: What I Tell Patients Across My Desk

• Daily maintenance: 250-500 mg is plenty. Your body can't use more.
• During illness: 1,000 mg every 6-8 hours for 2-3 days max. Then drop back to maintenance.
• Skip the megadoses: Above 2,000 mg daily increases kidney stone risk with no proven additional benefit.
• Check your meds: Vitamin C interacts with more drugs than people realize.

Mark, my software developer patient? We dropped him to 500 mg daily, added an iron supplement (ferrous bisglycinate—gentler on the stomach), and retested in 3 months. His ferritin normalized, and he still hasn't gotten sick. Sometimes the right dose isn't the biggest dose—it's the smartest one.

Disclaimer: This information is for educational purposes and doesn't replace personalized medical advice. Always consult with your healthcare provider before starting any supplement regimen.