Pharmaceutical drugs often react with foods and supplements, including vitamin C. And as we have seen, excessive vitamin C intake can interact with some nutritional compounds. Sometimes these interactions are beneficial such as increasing the absorption of minerals. And sometimes it leads to problems such as iron overload or copper and B12 deficiencies.
Another problem that is often overlooked is the interference of laboratory tests by excessive intake of vitamin C. For example, excessive vitamin C intake may lead to false high or low bilirubin levels depending on the assay test being used. Lactate dehydrogenase, cholesterol and triglyceride levels will read erroneously low. Aspartate aminotransferase levels may read erroneously high.
The National Institute of Health (NIH) reported on a case in which a woman with unexplained anemia was taking 2,000mg of vitamin C daily. When tested for occult blood in the stool repeated tests showed negative results. The woman discontinued taking the vitamin C for 4 days, and when retested stool samples tested positive for blood. It was also found that taking 750mg of vitamin C daily can interfere with detecting blood in stool and urine.
Vitamin C interferes with several glucose tests, including tests diabetics use at home. Urinary glucose test strips will test false positive with as little as 2,000mg of vitamin C daily. Home test strips can show normal blood glucose readings even when glucose levels are elevated or with as little as 2g of vitamin C daily. Laboratory glucose tests may show erroneously low glucose levels with excessive vitamin C intake.
To decrease the risk of false laboratory readings it is recommended that all supplements be stopped at least 48 hours before having any lab work done.
Below is a link from the NIH and a portion of the article that discusses the interactions of vitamin C with drugs and supplements, and interference with laboratory tests.
Interactions with Drugs
Acetaminophen (Tylenol): Vitamin C may increase adverse effects associated with acetaminophen.
Antacids: Vitamin C may increase adverse effects associated with aluminum-containing antacids such as aluminum hydroxide (Maalox, Gaviscon).
Aspirin: Vitamin C may increase blood levels and adverse effects of aspirin, whereas aspirin may decrease blood levels of vitamin C.
Barbiturates: The effects of vitamin C may be decreased by barbiturates including phenobarbital (Luminal, Donnatal), pentobarbital (Nembutal), or secobarbital (Seconal).
Fluphenazine (Permitil, Prolixin): Vitamin C supplementation may decrease levels of the drug fluphenazine in the body.
HIV medications (protease inhibitors): Concomitant administration of high doses of vitamin C can reduce steady-state indinavir plasma concentrations.
Levodopa (Dopar, Larodopa): There is limited case report evidence that high dose vitamin C may reduce side effects of levodopa therapy such as nausea or malcoordination.
Nicotine: Nicotine products such as cigarettes, cigars, chewing tobacco, or nicotine patches may decrease the effects of vitamin C.
Oral contraceptives/estrogens: Oral estrogens may decrease the effects of vitamin C in the body. When taken together, vitamin C may increase blood levels of ethinyl estradiol.
Tetracyclines: The effects of vitamin C may be decreased by tetracycline antibiotics such as doxycycline (Vibramycin), minocycline (Minocin), or tetracycline (Sumycin).
Warfarin (Coumadin): Vitamin C in high doses appears to interfere with the blood thinning effects of Warfarin by lowering prothrombin time (PT) as noted in case reports in the 1970s. Complications have not been reported (such as increased blood clots).
Interactions with Herbs and Dietary Supplements
Iron: When taken together, vitamin C may increase the absorption of iron in the gastrointestinal tract, although this effect appears to be variable and may not be clinically significant.
Lutein: Vitamin C may increase absorption of lutein vitamin supplements.
Vitamin B12 (cobalamin, cyanocobalamin): Large doses of vitamin C may interfere with the absorption and metabolism of vitamin B12.
Interactions with Laboratory Tests
Bilirubin: Vitamin C supplements may cause false increases in tests of blood bilirubin levels.
Carbamazepine levels: Vitamin C supplements may cause false increases in blood carbamazepine levels.
Creatinine: Vitamin C supplements may cause false increases in blood creatinine levels.
Glucose: Vitamin C supplements may interfere with the accuracy of blood glucose tests.
LDH (lactose dehydrogenase): Vitamin C may cause a false decrease in blood LDH levels.
Prothrombin time (PT): Vitamin C in high doses appears to interfere with the blood thinning effects of warfarin by lowering prothrombin time (PT), as noted in case reports in the 1970s. Complications have not been reported (such as increased blood clots).
SGOT (glutamic oxaloacetic transaminase): Vitamin C supplements may cause false increases in blood SGOT levels.
Stool occult blood (guaiac): Vitamin C supplements can cause false-negative stool occult blood tests, within 48-72 hours after vitamin C ingestion.
Theophylline levels: Vitamin C supplements may cause false decreases in blood theophylline levels.
Uric acid levels: Vitamin C supplements may cause false increases in blood uric acid levels.
Urinary acetaminophen (Tylenol): Vitamin C supplements can cause false-negative urine acetaminophen tests.
Urinary glucose: Vitamin C supplements can cause false-positive urinary glucose results with the cupric sulfate reagent test and false-negative urinary glucose results with the glucose oxidase test, within 48-72 hours after vitamin C ingestion.