Influenza prophylaxis in the long-term care facility: a case-control study of the risk factors for adverse drug reactions to amantadine.

BACKGROUND: Amantadine hydrochloride remains an inexpensive means of influenza A prophylaxis, but it is reported to have a high incidence of adverse drug reactions (ADRs) in residents of long-term care facilities (LTCFs) compared with newer, more expensive drugs.
OBJECTIVE: This study attempted to determine the effects of poor renal function on the rate of ADRs and any other variables on the tolerability of prophylaxis in this population. This would allow a high-risk subset of LTCF residents to be identified before prophylaxis, thus decreasing the risk for ADRs from amantadine.
METHODS: In this retrospective case-control study, a course of standardized low-dose (100-mg/d tablets) amantadine prophylaxis was ordered for all 242 residents of Ten Broeck Commons LTCF in Lake Katrine, New York, without influenza A for 14 days. Chart data of residents who developed ADRs (ADR group) were compared with those of a selected group who did not (control group). Residents' age, sex, renal function (blood urea nitrogen, serum creatinine, and creatinine clearance), dementia diagnosis, and number and classes of medications were compared.
RESULTS: The ADR group comprised 25 residents (21 women, 4 men; mean [SD] age, 84.8 [8.4] years); the control group, 29 residents (23 women, 6 men; mean [SD] age, 85.7 [7.5] years). The development of ADRs was not associated with differences in age, sex, renal function, or any medical condition, including measured, preexisting mental status changes between the groups. The overall risk for ADRs in the 242 residents was highest between days 8 and 14 of prophylaxis (17 residents [7.0%]) compared with the first 7 days (8 residents [3.3%]). Acute confusion was the most common ADR. All ADRs resolved on cessation of treatment.
CONCLUSIONS: No preexisting medical condition was statistically associated with an increased risk for ADRs, but an association with the number of days of prophylaxis was observed. By shortening prophylaxis to 7 days, the ADR risk may be lowered to be commensurate with more expensive medications.