Texas is an employment at will state and you can be terminated for any reason or no reason at all in most circumstances. If they fired you for failing a drug test, you really don't have any recourse. Now, you do mention that it turned out to be a work injury - if you think the termination may have been for filing a worker's compensation claim, then you might have a retaliation claim.
If you are a member of a union or have a specific contract of employment, then you may have some additional rights. You should look into a consultation with an employment lawyer in your area who can go through all of the wrinkles of your factual circumstances and help you determine what rights you may have.
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