An employer can establish attendance and leave policies that are uniformly applied to all employees, regardless of disability, but may not refuse leave needed by an employee with a disability if other employees get such leave.
An employer also may be required to make adjustments in leave policy as a reasonable accommodation. The employer is not obligated to provide additional paid leave, but accommodations may include leave flexibility and unpaid leave.
A uniformly applied leave policy does not violate the ADA because it has a more severe effect on an individual because of his/her disability. However, if an individual with a disability requests a modification of such a policy as a reasonable accommodation, an employer may be required to provide it, unless it would impose an undue hardship.
For additional information, take a look at the following resources:
FAQ: What are my employer's obligations after I return from medical leave? (What kind of documentation can the employer ask for when I return from medical leave?)
Fact Sheet: Work-Leave, the ADA, and the FMLA