Drug Interaction Report

MONITOR CLOSELY: Coadministration with nonsteroidal anti-inflammatory drugs (NSAIDs) may increase serum lithium levels and induce toxicity in some patients. The exact mechanism of interaction is unknown, but is thought to involve inhibition of renal prostaglandin synthesis by NSAIDs, resulting in decreased renal blood flow and lithium excretion. There have been numerous published reports of lithium toxicity, including severe cases, following the introduction of various NSAIDs including diclofenac, ibuprofen, indomethacin, ketorolac, mefenamic acid, piroxicam, and COX-2 inhibitors. However, pharmacokinetic studies have been somewhat inconsistent, with no significant effects reported for benoxaprofen and etodolac, and up to a 58% increase in serum lithium levels reported for indomethacin. Both decreased serum concentrations of lithium and no effect have been reported for sulindac and aspirin. The interaction is apparently subject to marked interpatient variability.

MANAGEMENT: Given the narrow therapeutic index of lithium, caution is advised during coadministration with NSAIDs, particularly in the elderly or patients with other risk factors (e.g., sodium restriction; renal impairment; congestive heart failure; dehydration; concomitant use of diuretics, ACE inhibitors, or angiotensin II receptor antagonists). Close monitoring for clinical signs of lithium toxicity is recommended if concomitant treatment with NSAIDs is required. Serum lithium levels should be checked every 4 to 5 days after starting an NSAID until the extent of any potential interaction can be evaluated. A reduction in lithium dosage may be needed in some cases. Renal function should also be monitored regularly. Patients should be advised to seek medical attention if they experience potential signs and symptoms of lithium toxicity such as drowsiness, dizziness, confusion, muscle weakness, vomiting, diarrhea, polydipsia, polyuria, tinnitus, tremor, ataxia, and blurred vision.

MONITOR: Famotidine may cause QTc prolongation. Theoretically, coadministration with other agents that can prolong the QT interval may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. According to the manufacturer, prolongation of the QT interval has been reported very rarely in patients with impaired renal function whose dose/dosing interval of famotidine may not have been adjusted appropriately. In general, the risk of an individual agent or a combination of these agents causing ventricular arrhythmia in association with QT prolongation is largely unpredictable but may be increased by certain underlying risk factors such as congenital long QT syndrome, cardiac disease, and electrolyte disturbances (e.g., hypokalemia, hypomagnesemia). In addition, the extent of drug-induced QT prolongation is dependent on the particular drug(s) involved and dosage(s) of the drug(s).

MANAGEMENT: Caution and clinical monitoring are recommended if famotidine is used in combination with other drugs that can prolong the QT interval. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope.

H2 antagonists may alter the pharmacokinetic disposition of some nonsteroidal anti-inflammatory drugs (NSAIDs), resulting in increased or decreased plasma concentrations. Data have been varied, even for the same NSAID. The mechanism may involve inhibition of metabolism, changes in gastric pH resulting in altered absorption, and/or reduced urinary elimination of the affected NSAIDs. Statistically significant changes have been small and of limited clinical significance when interactions have been observed.

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.

GENERALLY AVOID: The concurrent use of aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs) and ethanol may lead to gastrointestinal (GI) blood loss. The mechanism may be due to a combined local effect as well as inhibition of prostaglandins leading to decreased integrity of the GI lining.

MANAGEMENT: Patients should be counseled on this potential interaction and advised to refrain from alcohol consumption while taking aspirin or NSAIDs.

MONITOR: One study has suggested that caffeine withdrawal may significantly increase blood lithium levels. The mechanism may be involve reversal of a caffeine-induced increase in renal lithium excretion.

MANAGEMENT: When caffeine is eliminated from the diet of lithium-treated patients, caution should be exercised. When caffeine consumption is decreased, close observation for evidence of lithium toxicity and worsening of the psychiatric disorder is recommended. Patients should be advised to notify their physician if they experience symptoms of possible lithium toxicity such as drowsiness, dizziness, weakness, ataxia, tremor, vomiting, diarrhea, thirst, blurry vision, tinnitus, or increased urination.

H2 antagonists may reduce the clearance of nicotine. Cimetidine, 600 mg given twice a day for two days, reduced clearance of an intravenous nicotine dose by 30%. Ranitidine, 300 mg given twice a day for two days, reduced clearance by 10%. The clinical significance of this interaction is not known. Patients should be monitored for increased nicotine effects when using the patches or gum for smoking cessation and dosage adjustments should be made as appropriate.