No efficacy use now reaches grade A under the current grading system. The strongest supported signals are gout or serum uric acid reduction and symptom-management during cancer treatment, while common-cold, cardiovascular, cognition, iron, and wound-healing claims are more mixed, and eye-health prevention looks unsupported. Much of the literature still relies on combination antioxidant trials, which muddies interpretation.
What the research says
Each row is a different health effect. A means the support looks strongest, B likely helpful, C mixed or modest, and — not enough human evidence yet.
Papers are individual studies. Research reviews summarize evidence from multiple studies, such as systematic reviews and meta-analyses.
B · Likely helpful
B
Gout & serum uric acid
3 of 5 reviews positive. Vitamin C modestly but consistently reduces uric acid in the blood. Less powerful than allopurinol — not a replacement for gout-specific medication.
18 papers · 7 research reviews
C · Mixed or limited evidence
C
Common cold & upper respiratory infection
7 of 8 reviews positive. Modest reduction in cold duration (~8% adults, 14% children). Roughly halves cold incidence in marathoners, skiers, and soldiers under physical stress.
19 papers · 8 research reviews
C
Cardiovascular disease prevention & mortality
17 reviews usable. Modest blood-pressure reduction in type 2 diabetes and improved blood-vessel function. Primary cardiovascular prevention trials in healthy populations are largely null.
47 papers · 24 research reviews
C
Cognitive decline & dementia prevention
6 reviews usable. Most are mixed or null. Possible signal in observational dietary-intake data; supplementation trials less consistent.
27 papers · 7 research reviews
C
Wound healing & post-surgical recovery
5 of 10 reviews positive on pressure ulcer healing and post-surgical recovery, often co-administered with zinc. Effect strongest in deficient patients.
19 papers · 11 research reviews
C
Cancer adjuvant (chemotherapy & radiation side effects)
5 reviews on oral mucositis, fatigue, and quality-of-life during chemotherapy. Symptom-management only — not anti-cancer evidence. Most reviews mixed-positive.
16 papers · 7 research reviews
C
Iron absorption & iron deficiency anemia
Mechanistic absorption boost is real (~3–4×) but clinical hemoglobin and ferritin outcomes are inconsistent. Recent meta-analysis (n=905) showed no significant Hb or ferritin improvement.
24 papers · 6 research reviews
C
Cataracts & age-related macular degeneration
2 of 3 reviews null. AREDS2 vitamin C arm did not slow cataract or AMD progression. Most cataracts trials use combination antioxidant formulas where vitamin C effect cannot be isolated.
16 papers · 3 research reviews
Quick facts
Dose, form and timing based on trials in the database.
Typical dose
200–1,000 mg/day for general supplementation. Common cold therapeutic doses cluster at ≥1 g/day at symptom onset. RDA is 75–90 mg/day from food
Best taken
With water; iron-absorption boost requires co-administration with iron-containing meal. Divided doses (twice daily) for ≥1 g/day to reduce stomach upset and maintain plasma levels — the body excretes excess
Form
Plain ascorbic acid is the standard and what most trials use. Buffered forms (sodium ascorbate, calcium ascorbate / Ester-C) are gentler on stomach but clinically similar. Liposomal claims higher bioavailability but trial evidence is limited
Common side effects
Stomach upset (diarrhea, nausea, cramps) at doses ≥2 g/day. Generally very well-tolerated at typical doses. Buffered forms reduce stomach upset
Drug interactions
May reduce chemotherapy efficacy at high doses (especially methotrexate, bortezomib). Enhances non-heme iron absorption — relevant caution for people with iron-overload disorders. May reduce statin efficacy at megadoses (≥3 g/day)
Lab test interference
High-dose vitamin C (≥1 g/day) interferes with glucose meter readings, urine glucose tests, and fecal occult blood tests. Stop supplementation 24–72 hours before relevant testing
Megadose caution
Doses ≥3 g/day linked to increased oxalate kidney stone risk in susceptible individuals. Some observational data link >300 mg/day supplementation with increased cardiovascular risk in specific populations — flagged but not conclusive
Iron absorption nuance
Boosts non-heme iron absorption ~3–4× when co-administered with an iron source. But clinical trials measuring hemoglobin and ferritin show inconsistent benefit — mechanistic effect doesn't always translate to better iron status
Quality-verified products
Products that pass third-party quality testing.
Product quality data coming soon
Product recommendations are being built from USP Verified, NSF Certified, and Labdoor test data. Products will only appear here after the supplement itself clears the evidence bar above — and only if the product passes third-party purity testing.
How we graded Vitamin C
Grades come from 179 unique research papers indexed from PubMed — including 32 meta-analyses, 33 systematic reviews, 5 Cochrane reviews, and 96 randomized trials. Each supplement is graded by health effect using a certainty × effect matrix applied across PubMed evidence per health effect. Cochrane reviews and high-quality systematic reviews carry the most weight. From abstract-only data, "high certainty" is rare — most A grades come from moderate certainty plus consistent meaningful-positive evidence. See Methodology for full detail.