Not likely if they paid you for the time that you were suspended. They relied on a drug test. Drug tests are objective tests and sometimes come back with false positives and false negatives. They did the right thing by rehiring you. Given that CO is an at will employment state, you will likely tick off your employers by suing them for making an innocent mistake and then they will fire you. I would simply let this go.
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Of course you can sue. You can sue anyone for any thing at any time. Unfortunately, in your case it is unlikely that you can win.
Can you prove that the employer knew that you had passed the drug test and did not simply make a mistake? What sort of damages have you actually suffered? If you cannot establish that it was more than a mistake and that you suffered actual damages then you have no case. The employer paid you for the time you were off and reinstated you to your job. That is likely all that you are entitled to receive.
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In your question, you indicate that you were paid for the suspension time. Assuming your personnel file was also corrected, and you do not experience retaliation, it is unlikely that you will be able to establish damages sufficient to prevail on a claim. However, you should consult with an attorney to determine whether you may have viable claims. Please also remember that most claims will be forfeited unless they are brought within specific time periods. Some claims may also require you to file a complaint internally (with the employer) or with an administrative body (such as the EEOC or DOL). Again, it is really impossible to say with certainty whether you have claims you should pursue or not until a full consultation is done by an attorney.
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