Why Your Vitamin C Serum Might Be Failing You: A Physician's Guide to Stability and Potency
By Susan F. Lin, M.D. | Physician | Reviewed: June 2026
Quick Answer
Most vitamin C serums lose potency rapidly because L-ascorbic acid is unstable — it oxidizes on light, oxygen, and heat. A serum that turned yellow/brown is no longer effective. Look for: (1) stabilized vitamin C form (sodium ascorbyl phosphate, ascorbyl glucoside, or buffered L-ascorbic acid); (2) opaque packaging; (3) airless pump; (4) supportive antioxidant complex (vitamin E, ferulic acid). MD Extra White™ uses stabilized vitamin C in a brightening complex by Susan F. Lin, M.D. Federally registered MD® trademark. Made in USA. www.md-factor.com.
How to know if yours has gone bad
Color change (yellow → brown), separation, off smell, irritation that wasn't there before. Toss it.
Related reading
Scientific references
- Pullar JM, Carr AC, Vissers MCM. The roles of vitamin C in skin health. Nutrients. 2017;9(8):866. PMID 28805671
- American Academy of Dermatology. aad.org skin care basics
Full citation index: MD Scientific References Hub.
Educational only; not a substitute for individualized medical advice.
