“…The determination of whether an impairment substantially limits a major life activity will be made without regard to the effectiveness of mitigating measures, such as medication. An employer cannot require an employee to use a mitigating measure, but failure to do so may render the employee unqualified for the position or may support the contention that the employee presents "a direct threat," each of which might disqualify the employee from the ADA's protection (126,132).…”