I don't think that "a lot" is an accurate measure, more like some. According to this
article from 2014, urine testing revealed about 5.5% positive for at least one illicit drug. There were almost 4200 samples tested, with 279 positive results, so I think a valid sample size. That being said, there appeared to be a concentration in a single geographic region, which may have skewed the results.
The 2008 CF Health and Lifestyle Survey put the rate at less than 10 percent, based on anonymous disclosure, which matched the US rate. I don't have access to a more current version.
No zero tolerance programme ever achieves zero. Rest assured that we don't have legions of addicts at work that you have to watch out for. In addition, take comfort that those who use are quite regularly found out and dealt with.
Notwithstanding the above, I think alcohol is the most widely used substance in the CF. In any regard, it is everyone's responsibility to look out for each other and help folks who may be going down the wrong road.