First, trademarks do not have to be registered to be offered protection under state and federal trademark laws so, even if a mark is not registered, it does not mean the trademark is not being used.
You can check out the Lanham Act's (15 US Code) section 1127 to read about abandonment of a mark.
A mark may not be abandonned even if it is not currently being used on a product. A company who owns the trademark may be making product revisions or may be experiencing financial problems (ie. bankruptcy) which is putting a temporary halt on production.
In addition, old products bearing the trademark may still be in the marketplace and your use of this "abandonned" mark may lead to consumer confusion which could result in an infringement action.
I agree with FlyingRon that using a "drug name" adds to the legal complications you may face.
Unless you have a legitimate reason for using this particular trademark, I suggest you find a unique trademark of your own to identify your product. And you would be wise to review all of the particulars of your situation with an IP attorney in your area, before investing a lot of time, energy and money into your venture, to help you avoid as much as possible all legal risks.
Good luck.