Drug Interaction Report

MONITOR: Centrally-acting anticholinergic agents may antagonize the therapeutic effects of neuroleptic agents. Although these drugs have been used together clinically, the possibility of increased risk of adverse effects such as central nervous system depression and tardive dyskinesia should also be considered. In addition, excessive anticholinergic effects may occur in combination use, which can result in paralytic ileus, hyperthermia, heat stroke, and the anticholinergic intoxication syndrome. Peripheral symptoms of anticholinergic intoxication commonly include mydriasis, blurred vision, flushed face, fever, dry skin and mucous membranes, tachycardia, urinary retention, and constipation. Central symptoms may include memory loss, disorientation, incoherence, hallucinations, psychosis, delirium, hyperactivity, twitching or jerking movements, stereotypy, and seizures. In hot weather, the risk of hyperthermia and heat stroke should be considered, as neuroleptic agents can interfere with temperature regulation in the hypothalamus while anticholinergic agents tend to inhibit peripheral sweating mechanisms.

MANAGEMENT: Caution is advised if anticholinergic agents are used with neuroleptic agents, particularly in the elderly and those with underlying organic brain disease, who tend to be more sensitive to the central anticholinergic effects of these drugs and in whom toxicity symptoms may be easily overlooked. Prophylactic administration of anticholinergic agents is sometimes given clinically during neuroleptic therapy for drug-induced parkinsonism or extrapyramidal symptoms but may not always be appropriate. Patients prescribed this combination should be advised to notify their physician promptly if they experience potential symptoms of anticholinergic intoxication such as abdominal pain, fever, heat intolerance, blurred vision, confusion, and hallucinations. Ambulatory patients should be counseled to avoid activities requiring mental alertness until they know how these agents affect them. A dosage reduction in one or both drugs may be necessary if excessive adverse effects develop. During hot weather, patients should avoid prolonged sun exposure and intense physical exertion and maintain adequate fluid intake.

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GENERALLY AVOID: Alcohol may exacerbate the adverse central nervous system effects of psychoactive drugs, including methylphenidate.

GENERALLY AVOID: Consumption of alcohol while taking certain sustained-release formulations of methylphenidate may cause rapid release of the drug, resulting in increased systemic levels of methylphenidate. In vitro studies have been conducted using Metadate CD 60 mg and Ritalin LA 40 mg capsules, as well as Concerta 18 mg tablet. At an alcohol concentration of 40%, an increase in the release rate of methylphenidate was observed in the first hour for Metadate CD and Ritalin LA, resulting in 84% and 98% of the methylphenidate being released, respectively. In contrast, there was no increased release of methylphenidate in the first hour for Concerta. These results are considered to be representative of the other available strengths of the corresponding product.

MANAGEMENT: Patients treated with methylphenidate should be advised to avoid alcohol or medications that contain alcohol.

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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.

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GENERALLY AVOID: Use of anticholinergic agents with alcohol may result in sufficient impairment of attention so as to render driving and operating machinery more hazardous. In addition, the potential for abuse may be increased with the combination. The mechanism of interaction is not established but may involve additive depressant effects on the central nervous system. No effect of oral propantheline or atropine on blood alcohol levels was observed in healthy volunteers when administered before ingestion of a standard ethanol load. However, one study found impairment of attention in subjects given atropine 0.5 mg or glycopyrrolate 1 mg in combination with alcohol.

MANAGEMENT: Alcohol should generally be avoided during therapy with anticholinergic agents. Patients should be counseled to avoid activities requiring mental alertness until they know how these agents affect them.

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Theoretically, grapefruit juice may increase the plasma concentrations of loratadine as it does other drugs that are substrates of the CYP450 3A4 enzymatic pathway. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. The clinical significance of this potential interaction is unknown. Reported interactions with potent CYP450 3A4 inhibitors like clarithromycin, erythromycin and ketoconazole have produced substantial increases in the area under the plasma concentration-time curve (AUC) of loratadine and its active metabolite, descarboethoxyloratadine, without associated changes in the overall safety profile of the drug.

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The recommended maximum number of medicines in the 'antihistamines' category to be taken concurrently is usually one. Your list includes two medicines belonging to the 'antihistamines' category:

• diphenhydramine
• loratadine

Note: In certain circumstances, the benefits of taking this combination of drugs may outweigh any risks. Always consult your healthcare provider before making changes to your medications or dosage.