Aspirin (Acetylsalicylic acid). Aspirin may lower vitamin C level in blood, tissue and white blood-cells. Taking an aspirin a day to prevent heart attacks and strokes causes blood loss via the digestive tract on the order of about a tablespoon per day which results in iron loss. A low serum vitamin C level can cause excessive gastric mucosal bleeding. Vitamin C combats aspirin induced hemorrhage and boosts iron absorption.
Bortezomib (chemotherapy). In animal-trials oral vitamin C hinders the tumor-destructive effects of protease-inhibitors.
Zytostatika (chemotherapy). In animal-trials the combined intake of Doxorubicin with a peri-enteral application of vitamin C (2g/kg body weight, i.v. or intra-peritoneal) could reduce the cardio-toxic side-effects of Antrazyklins and increase survival-time significantly. Thereby the zytotoxic effect of the Anthorazyklins wasn't reduced. Pharmacological in-vitro studies have further shown, that vitamin C increases the zytotoxic effects of anti-neoplastic substances like Cisplatin, Dacarbazin, Doxorubicin, Gemcitabin, Pactitaxel, Tamoxiflen and 5-Flourouracil (5-FU).
Barbiturates (central nervous system depressants). The effects of vitamin C may be decreased by barbiturates including phenobarbital (Luminal®, Donnatal®), pentobarbital (Nembutal®), or secobarbital (Seconal®).
Paracetamol (pain reliever, fever reducer). Vitamin C may increase adverse effects associated with acetaminophen or aluminum-containing antacids such as aluminum hydroxide (Maalox®, Gaviscon®).
Fluphenazine (antipsychotic drug). Vitamin C supplementation may decrease levels of the drug fluphenazine in the body.
Indinavir (anti-retroviral therapy for AIDS). Concomitant administration of high doses of vitamin C can reduce steady-state indinavir plasma concentrations.
Levadopa (Parkinson disease). There is limited case report evidence that high dose vitamin C may reduce side effects of levodopa therapy such as nausea or malcoordination.
Oral 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.
Antibiotics. The effects of vitamin C may be decreased by tetracycline antibiotics such as doxycycline (Vibramycin®), minocycline (Minocin®), or tetracycline (Sumycin®). Concomitant intake of vitamin C. can enhace bioavailability and elevate blood levels of tetracycline. Prolonged use of tetracycline may reduce blood levels of ascorbic acid and can interfere with the activity of vitamin C as well as folic acid, potassium, and vitamins B2, B6, R12, and K. Vitamin C may also protect against hepatic and renal toxicity occasionally associated with tetracycline therapy, as well as the much more common adverse effect of dental and oral discoloration.
Warfarin (blood thinning agent). Data from some, but not all, animal experiments and rare, unqualified case reports indicates that vitamin C, in high doses, might decrease functional level and therapeutic acdtivity of warfarin, but the cumulative body of evidence fails to support a well-founded, consistent, and generalizable conclusion. Findings from the four controlled trials involving large number of subjects indicate a lack of clinically significant interaction between warfarin and ascorbic acid, event at doses as high as 10g vitamin C per day.
Insulin (diabetes treatment, technically orthomolecular substance). The cellular uptake of vitamin C is promoted by insulin and inhibited by hyperglycemia.