Clinical C 100t

Theoretically, vitamin B6 may have additive effects when used with antihypertensive drugs.<br> Clinical research shows that vitamin B6 can decrease systolic blood pressure in hypertensive patients (30859,82959,83093).

High doses of vitamin B6 may reduce the levels and clinical effects of phenytoin.<br> Preliminary clinical evidence suggests that vitamin B6 200 mg daily can reduce plasma levels of phenytoin, possibly by increasing metabolism. It is not known whether lower doses have any effect. Advise people taking phenytoin to avoid high doses of vitamin B6 (3046,10801).

High doses of vitamin B6 may reduce the levels and clinical effects of phenobarbital.<br> Preliminary clinical evidence suggests that vitamin B6 200 mg daily can reduce plasma levels of phenobarbital, possibly by increasing metabolism. It is not known whether lower doses have any effect. Advise people taking phenobarbital to avoid high doses of vitamin B6 (3046,10801).

Theoretically, vitamin B6 might increase the photosensitivity caused by amiodarone.<br> Despite initial case reports suggesting that pyridoxine may have a protective effect against amiodarone-induced photosensitivity, preliminary clinical research suggests that pyridoxine may actually exacerbate this adverse effect (8892,8893).

Vitamin B6 may increase the metabolism of levodopa when taken alone, but not when taken in conjunction with carbidopa.<br> Vitamin B6 (pyridoxine) enhances the metabolism of levodopa, reducing its clinical effects. However, this interaction does not occur when carbidopa is used concurrently with levodopa (Sinemet). Therefore, it is not likely to be a problem in most people (3046).

Theoretically, niacinamide may have additive effects when used with anticoagulant or antiplatelet drugs, especially in patients on hemodialysis.<br> Several cases of thrombocytopenia have been reported for hemodialysis patients treated with niacinamide 1 gram daily. Hemodialysis patients receiving niacinamide had almost a three-fold higher risk of developing thrombocytopenia when compared with those receiving placebo (25563,98940).

Niacinamide might increase the levels and adverse effects of carbamazepine.<br> Plasma levels of carbamazepine were increased in two children given high-dose niacinamide, 60-80 mg/kg/day. This might be due to inhibition of the cytochrome P450 enzymes involved in carbamazepine metabolism (14506). There is not enough data to determine the clinical significance of this interaction.

Niacinamide might increase the levels and adverse effects of primidone.<br> Case reports in children suggest niacinamide 60-100 mg/kg/day reduces hepatic metabolism of primidone to phenobarbital, and reduces the overall clearance rate of primidone (14506); however, there is not enough data to determine the clinical significance of this potential interaction.

Theoretically, selenium might prolong the sedating effects of barbiturates.<br> Laboratory research suggests that selenium can inhibit the hepatic metabolism of barbiturates (14601,14602). Selenium seems to prolong the sedative effect of pentobarbital in animal models (14601).

Selenium may have antiplatelet effects and may increase the risk of bleeding if used with anticoagulant or antiplatelet drugs.<br> Clinical research suggests that taking selenium 10 mcg/kg/day can increase bleeding times by increasing prostacyclin production, which inhibits platelet activity (14540). Other clinical research suggests that taking selenium 75 mcg daily, in combination with ascorbic acid 600 mg, alpha-tocopherol 300 mg, and beta-carotene 27 mg, reduces platelet aggregation (74406).

Theoretically, selenium might interfere with warfarin activity. <br> Animal research suggests that selenium can increase warfarin activity. Selenium might interact with warfarin by displacing it from albumin binding sites, reducing its metabolism in the liver, or by decreasing production of vitamin K-dependent clotting factors (14541). Selenium can also prolong bleeding times in humans by increasing prostacyclin production, which inhibits platelet activity (14540).

Theoretically, selenium supplementation may reduce the effectiveness of immunosuppressant therapy. <br> In vitro research and preliminary clinical evidence suggests that selenium may stimulate the immune system (74483,74445).

Contraceptive drugs might increase levels of selenium, although the clinical significance of this effect is unclear.<br> Some research suggests that oral contraceptives increase serum selenium levels in women taking oral contraceptives; however, other research shows no change in selenium levels (14544,14545,14546,101343). It is suggested that an increase could be due to increased carrier proteins, indicating a redistribution of selenium rather than a change in total body selenium (14545).

Selenium might reduce the beneficial effects of niacin on high-density lipoprotein (HDL) levels. <br> A combination of niacin and simvastatin (Zocor) effectively raises HDL cholesterol levels in patients with coronary disease and low HDL levels. Clinical research shows that taking a combination of antioxidants (vitamin C, vitamin E, beta-carotene, and selenium) along with niacin and simvastatin (Zocor) attenuates this rise in HDL, specifically the HDL-2 and apolipoprotein A1 fractions, by more than 50% in patients with coronary disease (7388,11537). It is not known whether this adverse effect is due to a single antioxidant such as selenium, or to the combination. It also is not known whether it will occur in other patient populations.

Theoretically, alpha-lipoic acid might decrease the effects of thyroid hormone drugs.<br> Animal research suggests that co-administration of thyroxine with alpha-lipoic acid reduces conversion into the active T3 form (8946).

Theoretically, the antioxidant effects of alpha-lipoic acid might alter the effectiveness of antitumor antibiotics.<br> The use of antioxidants like alpha-lipoic acid during chemotherapy is controversial. There are concerns that antioxidants could reduce the activity of antitumor antibiotic drugs, which work by generating free radicals (391). However, some researchers theorize that antioxidants might make chemotherapy more effective by reducing oxidative stress that might interfere with apoptosis (cell death) of cancer cells (14012,14013). More evidence is needed to determine what effect, if any, antioxidants such as alpha-lipoic acid have on chemotherapy involving antitumor antibiotics. Advise patients to consult their oncologist before using alpha-lipoic acid.

Theoretically, alpha-lipoic acid may have antiplatelet effects and may increase the risk of bleeding if used with anticoagulant or antiplatelet drugs.<br> In vitro, alpha-lipoic acid inhibits platelet aggregation (98682).

Theoretically, the antioxidant effects of alpha-lipoic acid might alter the effectiveness of alkylating agents. <br> The use of antioxidants like alpha-lipoic acid during chemotherapy is controversial. There are concerns that antioxidants could reduce the activity of chemotherapy drugs that generate free radicals (391). However, some researchers theorize that antioxidants might make chemotherapy more effective by reducing oxidative stress that might interfere with apoptosis (cell death) of cancer cells (14012,14013). More evidence is needed to determine what effect, if any, antioxidants such as alpha-lipoic acid have on chemotherapy. Advise patients to consult their oncologist before using alpha-lipoic acid.

Theoretically, taking alpha-lipoic acid with antidiabetes drugs might increase the risk of hypoglycemia.<br> Although some small clinical studies have suggested that alpha-lipoic acid can lower blood glucose levels (3545,3874,3875,3876,20490,20493,104650), larger clinical studies in patients with diabetes have shown no effect (20494,20495,20496,20498,90443,90445,103326). Additionally, co-administration of single doses of alpha-lipoic acid and glyburide or acarbose did not cause detectable drug interactions in healthy volunteers (3870).

Theoretically, hesperidin might inhibit P-glycoprotein-mediated drug efflux and potentially increase levels of drugs that are substrates of P-glycoprotein. <br> In vitro research shows that hesperidin can inhibit P-glycoprotein efflux (54908). This effect has not been reported in humans.

Theoretically, hesperidin may increase the levels and clinical effects of diltiazem.<br> Animal research suggests that hesperidin may enhance the bioavailability of diltiazem, increasing the plasma area under the curve of diltiazem by up to 65.3% (91761). This effect has not been reported in humans.

Theoretically, taking hesperidin with antihypertensive drugs might increase the risk of hypotension. <br> Some clinical and animal research shows that hesperidin can decrease blood pressure (54851,94543,98697,105278). However, other clinical research shows that hesperidin does not affect blood pressure (102315).

Theoretically, hesperidin may decrease the levels and clinical effects of celiprolol.<br> Animal research shows that concomitant use of hesperidin may reduce the plasma area under the curve of celiprolol by up to 75% (91760). This effect has not been reported in humans.

Theoretically, hesperidin might increase the levels and clinical effects of verapamil.<br> Animal research suggests that hesperidin may enhance the bioavailability of verapamil, increasing the plasma area under the curve of verapamil by 96.8% (91762). This effect has not been reported in humans

Theoretically, concomitant use with CNS depressants may cause additive sedative effects. <br> Animal studies show that hesperidin has sedative effects, due to opioid receptor activity (54841) and can increase sedation when used with diazepam (54789). This effect has not been reported in humans.

Theoretically, hesperidin may increase the risk of bleeding if used with anticoagulant or antiplatelet drugs. <br> Animal research suggests that hesperetin, a bioflavonoid aglycone derivative of hesperidin, may have antiplatelet activity (54822).

Theoretically, taking rutin with antidiabetes drugs might increase the risk of hypoglycemia.<br> Animal research suggests that rutin has hypoglycemic effects (105299).

Vitamin C might increase blood levels of estrogens. <br> Increases in plasma estrogen levels of up to 55% occur under some circumstances when vitamin C is taken concurrently with oral contraceptives or hormone replacement therapy, including topical products (129,130,11161). It is suggested that vitamin C prevents oxidation of estrogen in the tissues, regenerates oxidized estrogen, and reduces sulfate conjugation of estrogen in the gut wall (129,11161). When tissue levels of vitamin C are high, these processes are already maximized and supplemental vitamin C does not have any effect on estrogen levels. Increases in plasma estrogen levels may occur when patients who are deficient in vitamin C take supplements (11161). Monitor these patients for estrogen-related side effects.

Vitamin C can increase the amount of aluminum absorbed from aluminum compounds.<br> Research in animals and humans shows that vitamin C increases aluminum absorption, theoretically by chelating aluminum and keeping it in solution where it is available for absorption (10549,10550,10551,21556). In people with normal renal function, urinary excretion of aluminum will likely increase, making aluminum retention and toxicity unlikely (10549). Patients with renal failure who take aluminum-containing compounds such as phosphate binders should avoid vitamin C supplements in doses above the recommended dietary allowances.

Vitamin C can modestly reduce indinavir levels. <br> One pharmacokinetic study shows that taking vitamin C 1 gram orally once daily along with indinavir 800 mg orally three times daily reduces the area under the concentration-time curve of indinavir by 14%. The mechanism of this interaction is unknown, but it is unlikely to be clinically significant in most patients. The effect of higher doses of vitamin C on indinavir levels is unknown (11300,93578).

High-dose vitamin C might reduce the levels and effectiveness of warfarin.<br> Vitamin C in high doses may cause diarrhea and possibly reduce warfarin absorption (11566). There are reports of two people who took up to 16 grams daily of vitamin C and had a reduction in prothrombin time (9804,9806). Lower doses of 5-10 grams daily can also reduce warfarin absorption. In many cases, this does not seem to be clinically significant (9805,9806,11566,11567). However, a case of warfarin resistance has been reported for a patient who took vitamin C 500 mg twice daily. Cessation of vitamin C supplementation resulted in a rapid increase in international normalized ratio (INR) (90942). Tell patients taking warfarin to avoid taking vitamin C in excessively high doses (greater than 10 grams daily). Lower doses may be safe, but the anticoagulation activity of warfarin should be monitored. Patients who are stabilized on warfarin while taking vitamin C should avoid adjusting vitamin C dosage to prevent the possibility of warfarin resistance.

Theoretically, antioxidant effects of vitamin C might reduce the effectiveness of alkylating agents.<br> The use of antioxidants like vitamin C during chemotherapy is controversial. There is concern that antioxidants could reduce the activity of chemotherapy drugs that generate free radicals, such as cyclophosphamide, chlorambucil, carmustine, busulfan, and thiotepa (391). In contrast, some researchers theorize that antioxidants might make chemotherapy more effective by reducing oxidative stress that could interfere with apoptosis (cell death) of cancer cells (14012,14013). More evidence is needed to determine what effect, if any, antioxidants such as vitamin C have on chemotherapy.

Theoretically, the antioxidant effects of vitamin C might reduce the effectiveness of antitumor antibiotics.<br> The use of antioxidants like vitamin C during chemotherapy is controversial. There is concern that antioxidants could reduce the activity of chemotherapy drugs which generate free radicals, such as doxorubicin (391). In contrast, some researchers theorize that antioxidants might make chemotherapy more effective by reducing oxidative stress that could interfere with apoptosis (cell death) of cancer cells (14012,14013). More evidence is needed to determine what effects, if any, antioxidants such as vitamin C have on chemotherapy.

Vitamin C can increase levothyroxine absorption. <br> Two clinical studies in adults with poorly controlled hypothyroidism show that swallowing levothyroxine with a glass of water containing vitamin C 500-1000 mg in solution reduces thyroid stimulating hormone (TSH) levels and increases thyroxine (T4) levels when compared with taking levothyroxine alone. This suggests that vitamin C increases the oral absorption of levothyroxine, possibly due to a reduction in pH (102978).

Theoretically, vitamin C might decrease levels of fluphenazine. <br> In one patient there was a clinically significant decrease in fluphenazine levels when vitamin C (500 mg twice daily) was started (11017). The mechanism is not known, and there is no further data to confirm this interaction.

High-dose vitamin C might slightly prolong the clearance of acetaminophen.<br> A small pharmacokinetic study in healthy volunteers shows that taking high-dose vitamin C (3 grams) 1.5 hours after taking acetaminophen 1 gram slightly increases the apparent half-life of acetaminophen from around 2.3 hours to 3.1 hours. Ascorbic acid competitively inhibits sulfate conjugation of acetaminophen. However, to compensate, elimination of acetaminophen glucuronide and unconjugated acetaminophen increases (6451). This effect is not likely to be clinically significant.

Acidification of the urine by vitamin C might increase aspirin levels.<br> It has been suggested that acidification of the urine by vitamin C could increase reabsorption of salicylates by the renal tubules, and increase plasma salicylate levels (3046). However, short-term use of up to 6 grams daily of vitamin C does not seem to affect urinary pH or salicylate excretion (10588,10589), suggesting this interaction is not clinically significant.

Vitamin C might decrease the beneficial effects of niacin on high-density lipoprotein (HDL) cholesterol levels.<br> A combination of niacin and simvastatin (Zocor) effectively raises HDL cholesterol levels in patients with coronary disease and low HDL levels. Clinical research shows that taking a combination of antioxidants (vitamin C, vitamin E, beta-carotene, and selenium) along with niacin and simvastatin (Zocor) attenuates this rise in HDL, specifically the HDL-2 and apolipoprotein A1 fractions, by more than 50% in patients with coronary disease (7388,11537). It is not known whether this adverse effect is due to a single antioxidant such as vitamin C, or to the combination. It also is not known whether it will occur in other patient populations.

Acidification of the urine by vitamin C might increase choline magnesium trisalicylate levels.<br> It has been suggested that acidification of the urine by vitamin C could increase reabsorption of salicylates by the renal tubules, and increase plasma salicylate levels (3046,4531). However, short-term use of up to 6 grams daily of vitamin C does not seem to affect urinary pH or salicylate excretion (10588,10589), suggesting this interaction probably is not clinically significant.

Acidification of the urine by vitamin C might increase salsalate levels. <br> It has been suggested that acidification of the urine by vitamin C could increase reabsorption of salicylates by the renal tubules, and increase plasma salicylate levels (3046). However, short-term use of up to 6 grams/day vitamin C does not seem to affect urinary pH or salicylate excretion (10588,10589), suggesting this interaction probably is not clinically significant.

Concomitant use of tocotrienols with anticoagulant or antiplatelet agents might increase the risk of bleeding. However, this has not been reported in humans. <br> Taking tocotrienols orally inhibits experimentally-induced platelet aggregation in humans (3237,104429). Theoretically tocotrienols might increase the risk of bleeding if taken with antiplatelet or anticoagulant drugs. However, tocotrienols 400-800 mg daily have been used with aspirin and/or clopidogrel for 1 year with no clear cumulative antiplatelet effects and no reports of bleeding (104429).