Drug Interactions between calcium carbonate / melatonin / valerian / vitamin e and nalidixic acid

ADJUST DOSING INTERVAL: Administration with food may increase the absorption of calcium. However, foods high in oxalic acid (spinach or rhubarb), or phytic acid (bran and whole grains) may decrease calcium absorption.

MANAGEMENT: Calcium may be administered with food to increase absorption. Consider withholding calcium administration for at least 2 hours before or after consuming foods high in oxalic acid or phytic acid.

MONITOR: Oral caffeine may significantly increase the bioavailability of melatonin. The proposed mechanism is inhibition of CYP450 1A2 first-pass metabolism. After administration of melatonin 6 mg and caffeine 200 mg orally (approximately equivalent to 1 large cup of coffee) to 12 healthy subjects, the mean peak plasma concentration (Cmax) of melatonin increased by 137% and the area under the concentration-time curve (AUC) increased by 120%. The metabolic inhibition was greater in nonsmokers (n=6) than in smokers (n=6). The greatest effect was seen in subjects with the *1F/*1F genotype (n=7), whose melatonin Cmax increased by 202%. The half-life did not change significantly. The clinical significance of this interaction is unknown.

According to some authorities, alcohol may reduce the effect of melatonin on sleep. The mechanism of this interaction is not fully understood.

In addition, CYP450 1A2 inducers like cigarette smoking may reduce exogenous melatonin plasma levels. In a small clinical trial (n=8), habitual smokers had their melatonin plasma levels measured two times, each after a single oral dose of 25 mg of melatonin. They had smoked prior to the first measurement but had not smoked for 7 days prior to the second. Cigarette smoking significantly reduced melatonin plasma exposure (AUC) as compared to melatonin levels after 7 days of smoking abstinence (7.34 +/- 1.85 versus 21.07 +/- 7.28 nmol/L*h, respectively).

MANAGEMENT: Caution and monitoring are recommended if melatonin is used with inhibitors of CYP450 1A2 like caffeine or inducers of CYP450 1A2 like cigarette smoking. Consumption of alcohol should be avoided when taking melatonin.

GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of CNS-active agents. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

MANAGEMENT: Patients receiving CNS-active agents should be warned of this interaction and advised to avoid or limit consumption of alcohol. Ambulatory patients should be counseled to avoid hazardous activities requiring complete mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.

ADJUST DOSING INTERVAL: Oral preparations that contain magnesium, aluminum, or calcium may significantly decrease the gastrointestinal absorption of nalidixic acid. Absorption may also be reduced by sucralfate, which contains aluminum, as well as other polyvalent cations such as iron and zinc. The mechanism is chelation of nalidixic acid by polyvalent cations, forming a complex that is poorly absorbed from the gastrointestinal tract.

MANAGEMENT: When coadministration cannot be avoided, nalidixic acid should be dosed at least 2 hours before or 2 hours after polyvalent cation-containing products to minimize the potential for interaction.

MONITOR: Coadministration with certain quinolones may increase the plasma concentrations and pharmacologic effects of caffeine due to inhibition of the CYP450 1A2 metabolism of caffeine. Quinolones that may inhibit CYP450 1A2 include ciprofloxacin, enoxacin, grepafloxacin, nalidixic acid, norfloxacin, pipemidic acid, and pefloxacin (not all commercially available). In healthy volunteers, enoxacin (100 to 400 mg twice daily) increased systemic exposure (AUC) of caffeine by 2- to 5-fold and reduced its clearance by approximately 80%. Pipemidic acid (400 to 800 mg twice daily) increased AUC of caffeine by 2- to 3-fold and reduced its clearance by approximately 60%. Ciprofloxacin (250 to 750 mg twice daily) increased AUC and elimination half-life of caffeine by 50% to over 100%, and reduced its clearance by 30% to 50%. Norfloxacin 400 mg twice daily increased caffeine AUC by 16%, while 800 mg twice daily increased caffeine AUC by 52% and reduced its clearance by 35%. Pefloxacin (400 mg twice daily) has been shown to reduce caffeine clearance by 47%.

MANAGEMENT: Patients using caffeine-containing products should be advised that increased adverse effects such as headache, tremor, restlessness, nervousness, insomnia, tachycardia, and blood pressure increases may occur during coadministration with quinolones that inhibit CYP450 1A2. Caffeine intake should be limited when taking high dosages of these quinolones. If an interaction is suspected, other quinolones such as gatifloxacin, gemifloxacin, levofloxacin, lomefloxacin, moxifloxacin, and ofloxacin may be considered, since they are generally believed to have little or no effect on CYP450 1A2 or have been shown not to interact with caffeine.