Currently we don't know... but back several months ago we were having that problem and were removing 3 to 6 ads per day... sometimes per hour! And it just isn't against the drug companies it is against all the companies that do spam advertising...
As to why the software can't better determine... no clue, just as I'm not sure how the words were determined, etc. but I am looking into it! Just not something that can be fixed overnight!