If you are currently being treated with any of the following medications, you should not use vitamin C supplements without first talking to your healthcare provider.

Aspirin and Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)
Very limited research suggests that vitamin C may protect the stomach and intestines against injury from NSAIDs such as ibuoprofen. On the other hand, high doses of vitamin C (equal to or greater than 500 mg per day) may raise the blood levels of aspirin and other acidic medications.

Acetominophen
Vitamin C may decrease excretion of acetaminophen (a medication sold over the counter for pain and headache) in the urine, which may increase blood levels of this medication.

Diuretics, Loop
Animal studies suggest that vitamin C may amplify the effects of furosemide, which belongs to a class of medications known as loop diuretics.

Beta-blockers for high blood pressure
Vitamin C may decrease the absorption of propranolol, a medication that belongs to a class known as beta-blockers used for high blood pressure and other heart-related conditions. If taking vitamin C and a beta-blocker, therefore, it is best to take them at different times of the day.

Cyclosporine
Cyclosporine, a medication used for the treatment of cancer, may reduce blood levels of vitamin C.

Nitrate Medications for heart disease
The combination of vitamin C with nitroglycerin, isosorbide dinitrate, or isosorbide mononitrate reduces the occurrence of nitrate tolerance. Nitrate tolerance is when the body builds up a tolerance to the medicine so that it no longer has its desired effect. People taking nitrate-containing medications generally follow a 12 hours on, 12 hours off schedule to avoid this tolerance. Studies suggest that taking vitamin C along with nitrate medications may reduce the development of this tolerance.

Tetracycline
There is some evidence that taking vitamin C with the antibiotic tetracycline may increase the levels of this medication.

Warfarin
There have been rare case reports of vitamin C interfering with the effectiveness of this blood thinning medication. In recent follow up studies, no such association has been found with doses of vitamin C up to 1,000 mg per day. Because of these much earlier reports, however, some conservative clinicians suggest not exceeding RDA values of vitamin C (see earlier section entitled How To Take It). Whether taking recommended dietary amounts or larger quantities of vitamin C, anyone on warfarin must have their bleeding time measured regularly and followed closely using a value called an INR, measured at your doctor's office. If you take this blood thinner, any time you make a change to your diet, medications, or supplements, you must notify your physician.