Yes and No.  Mitigating measures reduce the effects of an impairment.  The ADA Amendments Act and the final regulations implementing the Act contain a non-exhaustive list of examples of mitigating measures, which includes medication, medical equipment and devices, prosthetic limbs, low vision devices, hearing aids, mobility devices, oxygen therapy equipment, use of assistive technology, physical therapy, psychotherapy and behavioral therapy.
The positive effects of mitigating measures in limiting the impact of an impairment  cannot be considered when determining whether someone has a disability.  Only the use of ordinary eyeglasses or contact lenses may be considered.  The ADA Amendments Act holds that the determination of disability must focus on whether the individual would be substantially limited in performing a major life activity absent the mitigating measure.  In most cases, this analysis will require focusing on the impairment’s effect before the individual started using the mitigating measure.  Alternatively, the determination could focus on what would happen if the individual stopped using the mitigating measure.
Despite the rule stated above, the negative effects of a mitigating measure can be taken into account in determining whether an individual is disabled.  Thus, if an individual experiences painful side effects from his or her medication, those side effects can be considered when determining whether that individual has a disability.