Does a doctor’s note excuse you from work?

Whether or not to accept a doctor's note as an excuse for absence from work is typically at the discretion of an employer. The fact is that companies can impose disciplinary action for absences even when an employee presents a a doctor's note.
If an employee qualifies for medical leave under the FMLA (Family Medical Leave Act), they must notify their employer of their status. While employees are not required to provide copies of medical records or otherwise disclose details about their injuries/illnesses, an employer does have the right to ask an employee to provide medical proof, typically a note from a doctor, that proves the existence of the problem.
Some injuries and illnesses prevent a worker from being able to perform their duties. In those cases, an employer must decide if if a temporary layoff or permanent dismissal is necessary. In such a case, a doctor's note might not make a difference in the employer's decision. The fact is that majority of states have "at will" employment which means that an employer has the right to fire an employee for any reason or no reason at all, as long as discrimination isn't a factor in the firing or it does not violate the terms of a union agreement or employment contract. As a result, an employers can fire an employee with or without a doctor's note.
That having been said, an employer cannot termiate an employee because they filed a workers' compensation claim, as long as they can perform their job duties. Additionally, if the injury/illness is legally documented to be a permanent disability, an employer must make "reasonable accomodations" to assist an employee if they can still perform the major reponsibilities of their job, according to the ADA (Americans with Disabilities Act).