Drug Interactions between linezolid and Mooredec

CONTRAINDICATED: Linezolid may potentiate the pressor response to sympathomimetic agents. Linezolid is a reversible, nonselective monoamine oxidase inhibitor (MAOI) and, as such, may enhance sympathomimetic effect by increasing norepinephrine storage in adrenergic neurons. The interaction may be more likely to occur with indirect- or mixed-acting sympathomimetics such as pseudoephedrine or ephedrine than with direct-acting agents like epinephrine, norepinephrine, and isoproterenol. In healthy normotensive subjects, coadministration of linezolid (600 mg every 12 hours for 3 days) and two doses of pseudoephedrine (60 mg each) or phenylpropanolamine (25 mg each) given 4 hours apart resulted in a mean maximum increase in systolic blood pressure of 32 mmHg and 38 mmHg, respectively, compared to baseline. The mean maximum systolic blood pressure during coadministration was also significantly increased compared to either agent alone. Heart rate was not affected. Maximum blood pressure levels were seen 2 to 3 hours after the second dose of phenylpropanolamine or pseudoephedrine, and returned to baseline 2 to 3 hours after peak.

MANAGEMENT: Unless blood pressure and clinical status can be closely monitored, linezolid should not be administered in combination with direct- or indirect-acting sympathomimetic agents (e.g., pseudoephedrine, phenylpropanolamine), vasopressive agents (e.g., epinephrine, norepinephrine), or dopaminergic agents (e.g., dopamine, dobutamine). If the combination is used, lower initial dosages of adrenergic agents such as dopamine or epinephrine are recommended, with careful titration to the desired response.

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GENERALLY AVOID: Coadministration of monoamine oxidase inhibitors (MAOIs) and antihistamines may result in additive central nervous system depressant effects. In addition, limited data suggest that MAOIs may potentiate and prolong the anticholinergic effects of antihistamines due to inhibition of catecholamine degradation, which may lead to overstimulation of the sympathetic nervous system. In one published report, a woman who had been on phenelzine 30 mg/day for six months developed irritability and visual hallucinations two months following the addition of cyproheptadine 2 mg at bedtime to treat phenelzine-induced anorgasmia. The hallucinations cleared over 48 hours following the discontinuation of her medications. In another published report, a patient developed delirium with symptoms of aggression, paranoia, and vivid auditory as well as visual hallucinations after two days of receiving diphenhydramine 300 mg/day and linezolid 600 mg every 12 hours. The patient also had tachycardia, very warm skin, and possibly blurred vision (as evidenced by constant squinting). Central anticholinergic intoxication and dopaminergic hyperactivity were suspected. Symptoms subsided over four days following the discontinuation of diphenhydramine, while linezolid was continued with no subsequent sequelae. In a third report, a patient developed visual hallucinations associated with confusion and disorientation after nine days of linezolid and antihistamine therapy, including dexchlorpheniramine and cetirizine for the first four days and hydroxyzine for the next five. Physical examination did not reveal any focal neurological signs, myoclonus or ataxia, and cerebral CT scan and EEG were within normal limits. Symptoms resolved two days after linezolid was discontinued.

MANAGEMENT: Prescribing antihistamines in combination with MAOIs or other agents that possess MAOI activity (e.g., furazolidone, linezolid, procarbazine) should generally be avoided. If concomitant treatment is unavoidable, patients should be counseled to avoid hazardous activities requiring 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. It may be appropriate to monitor some patients for increased anticholinergic effects (e.g., constipation, urinary retention, fever, heat intolerance, blurred vision, confusion, hallucinations, dizziness, palpitations, arrhythmias, syncope), since certain populations such as the elderly and those with underlying organic brain disease tend to be more sensitive to these effects and may be susceptible to anticholinergic intoxication. It should be noted that the manufacturers of many of the sedating antihistamines (e.g., chlorpheniramine, dexchlorpheniramine, diphenhydramine, pheniramine, promethazine) consider their use within 14 days of MAOIs to be contraindicated.

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