Hello! I am a licensed attorney, admitted to practice in state and federal court. I have a nearly 100% satisfaction rating so all that means is that you can count on me to help today. You may have Two causes of action here. The first cause of action that you may have is under FMLA. Under FMLA, an employee who is injured and needs to take time off work may be eligible for up to 12 weeks of unpaid, job-protected leave. The requirements are that the employer must have at least 50 employees, the employee must have worked for the employer at least 12 total months, and the employee must have worked at least 1,250 hours for the employer. If the employer learns that the employee may need time off from work, then they are required to offer FMLA and approve it if there is a condition that warrants it after receiving information. In your case, if you returned to work and they knew that you were still on medication and needed more time, then it is possible and probable that they should have offered you FMLA. Thus, you may be able to file a complaint with your department of labor.
In addition, you may have a claim against them for discrimination based on your disabilities. Under 42 USC 2000e-2, if you have a disability, then an employer may not treat you in a discriminatory manner. Now, what qualifies as a disability is up for debate. Things such as being blind, handicapped, or otherwise incapacitated may lead to an obvious disability classification. However, it is possible that your infection could be considered a disability, and if they forced you to take a drug test knowing you may be on medication for your disability, this could be discrimination.
This also appears to be a violation of the implied covenant (promise) of good faith and fair dealing. Under the Uniform Commercial Code Section 1-304, every contract comes with implied promises from both parties that they are going to be acting fairly toward one another. The failure of one party to act fairly toward the other can result in a breach of contract. To facilitate your situation, there’s a site that I’ve used in the past where you can find a good template for advising of a breach of contract (click here). It's a bit easier (and cheaper) than going through litigation and I have seen it be effective in the past. If you send this letter and they do not respond, then you may want to consider bringing a lawsuit after that because it will be clear they will not be willing to settle the issue.
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