Does Vitamin C Really Cause False Blood Glucose Readings? Not at an Amount I can Eat.

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Disclaimer: This experiment was only done on one person (myself) and is not fully randomized, placebo controlled, or blinded. Please take all conclusions with the appropriate skepticism.


Vitamin C is reported to falsely raise blood glucose readings in both fingerstick and continuous glucometers. I wanted to know if this was something that I (and others) should be concerned about, but all the reports I found were for intravenous vitamin C and I couldn’t get any data from the CGM companies. 

To get an answer, I tested it myself, monitoring blood glucose after ingesting 2-6g of vitamin C, 20-60 the FDA recommended daily amount (but known to be safe). I did not observe any consistent or sustained increase in blood glucose, so I conclude that ingested vitamin C has no measurable impact on my blood glucose measurements at any practical quantity.



When I first tried out the FreeStyle Libre CGM, I read through the manual and noticed an unusual warning:

“Taking ascorbic acid (vitamin C) while wearing the Sensor may falsely raise Sensor glucose readings. Taking salicylic acid (used in some pain relievers such as aspirin and some skin care products) may slightly lower Sensor glucose readings. The level of inaccuracy depends on the amount of the interfering substance active in the body.”

Vitamin C is an essential nutrient and present in tons of foods and most multivitamins. If it falsely raises glucose readings, that’s a huge flaw in the device.

I tried contacting Abbot, who confirmed that vitamin C can cause erroneously high glucose readings, but couldn’t point me to any studies or say how strong the effect was. I also contacted Dexcom and there support rep. said that “Vitamin C was not contraindicated for Dexcom CGMs,” but again couldn’t point me to any studies or say if they had or hadn’t run any tests.

Having failed at getting useful info directly from the CGM companies, I turned to the published literature. I found a number of reports of Vitamin C leading to high blood glucose readings (1, 23, 4, 5; not exhaustive, there are many more). From these, I learned the following:

  • All reports in scientific or medical journals were for patients taking intravenous vitamin C.
  • Vitamin C can be oxidized by glucose oxidase and glucose dehydrogenase, the enzymes used to detect glucose in most portable glucometers, including most fingerstick meters and both the FreeStyle Libre and Dexcom CGMs
  • The increase in reported blood glucose can be extremely large; in one case a women read 343 mg/dL on standard glucometer, while a blood test showed her actual blood glucose as 49 mg/dL.
  • The blood levels of vitamin C in these reports were extremely high, 1,000 – 5,000 µmol/L. This is much higher than the 100-200 µmol/L observed with regular oral vitamins.
  • There’s only one anecdotal report on a diabetes forum of someone who saw high CGM readings after taking oral vitamin C, with numerous comments from people claiming that they do not see this.

Given all this, it seemed unlikely that regular consumption of vitamin C would significantly affect blood glucose readings, but I was left with two questions:

  • Was there any measurable effect of ingested vitamin C, even if small?
  • Would mega-doses of vitamin C have a significant effect?
    • mega-doses of vitamin C were advocated by Linus Pauling (nobel prize winner in chemistry). His theory has since been debunked, but is unfortunately still used by many people.

Since I couldn’t find any direct studies of the effect of ingested vitamin C, I decided to do my own experiment. 


To quantify the effect of ingestion of vitamin C in blood glucose readings from Dexcom G6 and Freestyle Libre CGMs.


Procedure. At selected times of day, 2, 4, or 6g of vitamin C was ingested and blood glucose monitored using a continuous glucose monitor.

Measurements. Blood glucose was measured using a Dexcom G6 CGM and FreeStyle Libre 14 day CGM.

Data Processing & Visualization. Data was visualized using Tableau.

Medication. I took my normal morning, evening, and meal medications


set1, set2

Results & Discussion

Figure 1. Blood Glucose vs. time for each of four vitamin C ingestion experiments. Color represents data from Dexcom G6 (blue) and FreeStyle Libre (orange)

According to the NIH, the recommended amount of vitamin C is 65-90 mg/day and the tolerable upper limit is 2,000 mg/day. However, studies have shown no serious adverse effects at much higher amounts and >10,000 mg/day is prescribed for treatment of certain cancers with no ill effects. Given that information, I tested ingestion of 2,000, 4,000, and 6,000 mg of vitamin C. 

As can be seen in Figure 1, there was no significant rise in measured blood glucose in any of the four experiments that could be attributed to vitamin C intake. While here were transient increases in measured blood glucose in the first three measurements, they were not consistent in time or magnitude. In addition, all the reports of increases from intravenous vitamin C indicated that the effect was of long-duration (such that measured blood glucose was still elevated well after the patient arrived at the hospital or clinic). 

Based on this data and the fact that typical vitamin C consumption is much less than what I took in these experiments, I conclude that ingested vitamin C has no measurable impact on blood glucose measurements at any practical quantity.

Final Thoughts & Next Experiments

This was a very satisfying experiment. Pretty quick, answered a question I wasn’t able to find in the literature or from the CGM companies (though they probably knew the answer), and results were very clear. 

Next up:

  • Re-tune basal and bolus (meal) insulin doses
    • My routine has changed a lot due to working from home, changing doctors, and changing medication (due to insurance requirements). Plus, I was able to get a Dexcom G6 CGM, which is showing accuracy comparable to my blood glucose meter. Blood sugars are still good, but I think I can get them better.
    • This is going pretty well. As I expected, I had my basal:bolus ratio off. I’m steadily getting it corrected and it’s making my blood sugar a lot more stable.
  • Re-measure blood sugar impact of glucose and insulin; compare to previous data
    • While working from home, I’ve gained some weight (and hopefully muscle). This has resulted in a change in my insulin sensitivity. Not huge, but I need to remeasure to have an accurate baseline for future experiments.
  • Test the effect of some dietary supplements that have been reported to affect blood sugar in the literature, but where data insufficient or contradictory
    • Glutamate (reported to reduce post-prandial glucose, but magnitude and timing vary widely)
    • Others tbd. 

As always, if you have any questions, comments, suggestions, or are interested in joining in future experiments, please let me know in the comments or send a PM via the contact form or to

– QD

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22 thoughts on “Does Vitamin C Really Cause False Blood Glucose Readings? Not at an Amount I can Eat.

  1. My blood sugars stay fairly stable during the day but spike and rise while fasting at night so I have a higher fasting reading, higher than before going to bed. Any helps or ideas to bring that down?

    1. For me, I get higher fasting BG if I eat too late at night or too large a meal at dinner. I've taken to eating earlier as a result. I also take metformin to reduce release of endogenous glucose by my liver and a long-acting insulin, but I wouldn't recommend either of those unless you have diabetes and consult with your doctor.

  2. Linus Pauling's assistant did a ludicrous test whereby he irradiated hairless rats with UV until they developed skin cancer.
    They were then given small amounts of Vitamin C while still grossly irradiated, and of-course did not recover.
    This ridiculous farce somehow "debunked" vitamin C therapy.
    I would like to have seen his (the assistant) bank account before and afterward…


  3. Thank you for sharing your experiment, very fascinating indeed. I am new to CGMs but have a different experience. I am following the Linus Pauling Protocol and taking 20+ grams of vitamin C daily. I noticed that Freestyle Libre gave much higher (40-50 mg/dl) readings as compared to a finger stick glucometer. Once I discontinued the vitamin C, the Freestyle readings matched the finger stick (which was calibrated by a control solution). I love being able to monitor my glucose but need the therapeutic vitamin C. Maybe Dexcom’s CGM is less distorted by vitamin C. Guess the only way to really know, is to give it a try. Any comments? Thank you.

    1. Very interesting. A few thoughts/questions:
      1) I’m impressed you can take 20+g. I found the 6g dose to be highly unpleasant.

      2) I tested both the Dexcom and Libre and did not see an increase from vitamin C ingestion with either. Both companies warn the Vitamin C can raise readings, as well.

      3) The difference in our results could be due to quantities. The largest dose I took was 6g and my experiments only lasted 3 days. 20+g daily is way higher and closer to the intravenous quantities that showed increased readings in the literature.

      4) The other possibility is that the Libre has a significant sensor-to-sensor offset error. When you discontinued the Vitamin C and saw readings return to normal, was that all with the same sensor or did you change sensors?

      5) After stopping the Vitamin C, have you re-started and seen the effect return? If you stop/start on a regular basis, it would be very interesting to see CGM and fingerstick measurements taken at some regular cadence starting before and running through the first few days of the treatment to see if the discrepancy builds up over time (as blood concentration of Vit C increases).

      1. The only thing I’ve done is to reduce my Vitamin C to 500 mg.
        My system has been replaced, but its problem was not related to Vitamin C. The problem with the old one was holding a charge, so that has nothing to do with my problem. Obviously, I liked your comments about Vitamin C. I could try reducing the Vitamin C by 500 mg and see what happens.

  4. Thanx for your comments on Vitamin C. My Vitamin C problem is too-fold and I require large amounts for medical purposes. (1) I have macular degeneration and PreserVision Areds 2 contains 500 mg of Vitamin C per day. (2) I’m susceptible to UTIs and large amounts (1500 mg) per day keep these infections under control.
    I quit using finger sticks and went to a fingerless monitor, but its information states that anything above 500 mg per day will result in false high readings. This system is Libre2.
    How do I find a fingerless system that will accommodate my Vitamin C requirements?

    1. Have you tested your Libre2 to confirm that it gives high readings? I didn’t try taking Vitamin C every day, but I saw no effect from multi-gram doses several days in a row.

      All continuous/flash glucose monitors, what you call “fingerless” meters, have the same warning about vitamin C, so if you do confirm an effect, I don’t think there’s any way around it.

  5. I recently switched to the Libra 2 meter. This meter does seem to be very sensitive to the C that I take and within an hour is giving me HI readings even though a blood test meter says I am high in the 350 range not the HI that the libra 2 meter says.
    C normally does not stay long in your system. I take one that is time-released and lasts for approximately 12 hours per pill once it is in your system. Which may or may not be why the meter goes crazy.

  6. I have chronic bronchitis/rhinitis from allergies, and histamine intolerance. Vitamin C has helped previously but may have caused a bought of severe stomach distress. Today I went and had IV vit c, 1 gram. My Libre2 sensor reached 350 (from 110) within 20 minutes at 6:30 and has just now receded to 220 at 12:30. I’m wondering if I do only 500mg on the next infusion would it not go up so high? It’s hard to wrap my brain around the idea that it’s a false reading when looking at the high number!! My A1c is 6.8. Could this skew that number?

    1. I have the exact same problem with histamine and need high doses of Vitamin C. My bronchitis is much better with it. My Libre hit 350 when I received a 1g iv in December. I need another iv but would like to find a handheld monitor that isn’t affected. I am considering getting just a 500 mg iv just to keep me calm because of the reading! Would love to hear of you have taken a half dose. I am taking vit c licaps but the iv is amazing as it improved my lungs function by 30%

    2. Vitamin C shouldn’t affect A1C. That measures glycation of hemoglobin via a very different process than how CGMs work. There’s no literature or biologic mechanism I know of that would cause vitamin C to interfere.

  7. would hesperidin and diosmin at 1000 mg cause my blood sugar to go high? I am on trulicity and lantus at night. The trulicity has been working well until I started taking the hesperidin. Any ideas?

    1. No idea. The only references I can see on hesperidin claim it lowers BG. I don’t know if that’s true, but seems unlikely it raises BG. You might want to check if there are any fillers (e.g. dextrose) to see if that might be having an effect.

  8. I almost didn’t buy the KetoMojo monitor I wanted because I take about 4 gr Vitamin C a day, but I called to talk to them because their product manual states “Do not use this system if you are taking vitamin C (ascorbic acid in your blood > 3 mg/dL) since it could cause your glucose and ketone results to be incorrect.” I was all ready to order a lab blood test to check myself. When I called KetoMojo they responded that this pertains only to Vitamin C IVs, and stated that he himself also takes Vitamin C while using the monitor and it’s fine. So I suppose one could take a lab test to make sure they are under 3 mg/dL, but I used the KetoMojo older model for years while taking Vitamin C and didn’t notice anything unusual in my readings, and my glucose results were fairly close to when I did lab glucose testing.

  9. Thank you for this post. I no longer have to eye my pouch of Camu Camu with suspicion and circumspection!

  10. I don’t believe that the problem is that vitamin C actually raises blood sugar–the problem is that vitamin C interacts with the enzymes that measure blood sugar in continuous monitoring and fingerstick point of care instruments.

    A true test would be to check glucose using a point of care instrument like the freestyle Libre 2 or similar, then have blood simultaneously drawn to have a lab test, which uses different means to check the glucose level in the blood.

  11. I’ve been using the Freestyle Libre 2 for almost a year and since I started taking vitamin C daily this past week my glucose readings have been exceptionally high versus normally. When I check with a finger blood prick they are actually ok. So for me the vitamin C is for sure having an affect on my sensor readings.

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