Drug Interactions between calcium carbonate / melatonin / pyridoxine and methamphetamine

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.

GENERALLY AVOID: Alcohol may potentiate the central nervous system and cardiovascular effects of centrally-acting appetite suppressants. In one study, concurrent administration of methamphetamine (30 mg intravenously) and ethanol (1 gm/kg orally over 30 minutes) increased heart rate by 24 beats/minute compared to methamphetamine alone. This increases cardiac work and myocardial oxygen consumption, which may lead to more adverse cardiovascular effects than either agent alone. Subjective effects of ethanol were diminished in the eight study subjects, but those of methamphetamine were not affected. The pharmacokinetics of methamphetamine were also unaffected except for a decrease in the apparent volume of distribution at steady state.

MANAGEMENT: Concomitant use of centrally-acting appetite suppressants and alcohol should be avoided if possible, especially in patients with a history of cardiovascular disease. Patients should be counselled 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.

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.