Allopurinol is used to treat gout. Taking large doses of niacin might worsen gout and decrease the effects of allopurinol.

Gemfibrozil

• Lopid

Taking niacin along with gemfibrozil might increase the risk for muscle damage in some people. Use with caution.

Medications for diabetes

• Glyburide

• Glipizide

• Insulin

• Sitagliptin

• Empagliflozin

• Semaglutide

• Dapagliflozin

High doses of niacin might increase blood sugar levels. Taking niacin along with diabetes medications might reduce the effects of these medications. Monitor your blood sugar closely.

Medications for high blood pressure

• Amlodipine

• Ramipril

• Perindopril

• Metoprolol

• Losartan

• Hydrochlorothiazide

Niacin might lower blood pressure. Taking niacin along with medications that lower blood pressure might cause blood pressure to go too low. Monitor your blood pressure closely.

Medications that can harm the liver

• Acetaminophen / Paracetamol

• Methotrexate

• Atorvastatin

• Simvastatin

• Isoniazid

Niacin might harm the liver. Some medications can also harm the liver. Taking niacin along with a medication that can harm the liver might increase the risk of liver damage.

Medications that slow blood clotting

• Heparin

• Rivaroxaban

• Apixaban

• Dabigatran

• Clopidogrel

Niacin might slow blood clotting. Taking niacin along with medications that also slow blood clotting might increase the risk of bruising and bleeding.

Medications used for lowering cholesterol

• Bile acid sequestrants

Some medications called bile acid sequestrants can decrease how much niacin the body absorbs. This might reduce the effects of niacin. Take niacin and these medications at least 4-6 hours apart.

• Atorvastatin

• Rosuvastatin

• Simvastatin

• Pravastatin

• Fluvastatin

Taking niacin along with statins might increase the risk for muscle damage in some people. Use with caution.

Nicotine patch

• Nicoderm

Niacin can sometimes cause flushing and dizziness. Nicotine patches can also cause flushing and dizziness. Taking niacin and using a nicotine patch may increase the risk of flushing and dizziness.

Probenecid

• Benemid

Probenecid is used to treat gout. Taking large doses of niacin might worsen gout and decrease the effects of probenecid.

Sulfinpyrazone

• Anturane

Sulfinpyrazone is used to treat gout. Taking large doses of niacin might worsen gout and decrease the effects of sulfinpyrazone.

Thyroid hormone

The body naturally produces thyroid hormones. Niacin might decrease thyroid hormone levels. Taking niacin with thyroid hormone pills might decrease the effects of thyroid hormone.

• Jantoven

• Used for: blood thinning, clot prevention

Niacin might increase the blood thinning effects of warfarin and potentially increase the chance of bleeding. Your healthcare provider may need to monitor you more often if you take niacin along with warfarin. The dose of your warfarin medication may need to be changed.

Aluminum

Aluminum is found in most antacids. Vitamin C can increase how much aluminum the body absorbs. However, it is not clear if this interaction is a big concern. Take vitamin C two hours before or four hours after antacids.

Estrogens

• Oral contraceptive pill

• Estradiol

• Conjugated estrogens

• HRT combination products

Vitamin C might decrease how quickly the body gets rid of estrogens. Taking vitamin C along with estrogens might increase the effects and side effects of estrogens.

Fluphenazine

• Prolixin

Large amounts of vitamin C might decrease how much fluphenazine is in the body. Taking vitamin C along with fluphenazine might decrease the effectiveness of fluphenazine.

Indinavir

• Crixivan

Taking large amounts of vitamin C along with indinavir might decrease how much indinavir stays in the body. It's not clear if this interaction is a big concern.

Levothyroxine

• Synthroid, others

Taking vitamin C along with levothyroxine might increase how much levothyroxine the body absorbs. This can increase the amount of levothyroxine in the body and increase its effects and side effects.

Medications for cancer

• Cyclophosphamide

• Chlorambucil

• Melphalan

• Busulfan

• Temozolomide

Vitamin C is an antioxidant. There is some concern that antioxidants might decrease the effects of some medications used for cancer. If you are taking medications for cancer, check with your healthcare provider before taking vitamin C.

• Doxorubicin

• Bleomycin

• Daunorubicin

• Mitomycin

• Epirubicin

Vitamin C is an antioxidant. There is some concern that antioxidants might decrease the effects of medications used for cancer. If you are taking medications for cancer, check with your healthcare provider before taking vitamin C.

Niacin

Taking niacin with vitamin C and other antioxidants can decrease the effects of niacin on good cholesterol levels. It is unknown if vitamin C alone decreases the effects of niacin on good cholesterol levels.

Warfarin is used to slow blood clotting. Large amounts of vitamin C might decrease the effects of warfarin. Decreasing the effects of warfarin might increase the risk of clotting. Be sure to have your blood checked regularly. The dose of your warfarin might need to be changed.