You almost certainly cannot sue your employer. An employer is only liable for the deliberate or negligent acts of its employees committed in the course of employment. Since marijuana (other than properly prescribed medical marijuana in a handful of states) is illegal, it would not have been part of an employee's job to give out pot cookies; hence the employer is not liable for that act. If you know who the employee who gave it to you was, you could sue him or her IF you suffered some injury or cost (e.g. you required medical treatment)--the U.S. legal system provides compensation for actual injuries, costs, or losses, so if you were not harmed, you would not be able to recover even enough money to cover the cost of the lawsuit.