It's the name of a drug, and drug names are generally not capitalized unless they are part of a proper noun (like a brand name).
For example:
* Incorrect: I took a Hydrocodone for my pain.
* Correct: I took a hydrocodone for my pain.
However, if you are referring to a specific brand name like "Vicodin", then you would capitalize it:
* Correct: I took a Vicodin for my pain.