Dear Oliver, 

check this out if it helps you: 

"According to the Federal Aviation Administration and the Office of Aerospace Medicine, 11 percent of the 5,321 pilots involved in aviation accidents between 1990 and 2005 tested positive for drug use. Between 2004 and 2008, 37 percent of the 1,353 pilots who died in aviation accidents tested positive for drugs."

http://www.thedailybeast.com/articles/2014/07/23/busted-for-heroin-when-pilots-fly-high.html

The Center for Disease Controll will have data sets you can access to calculate prevalence estimates and incidence rates over time by occupation. That can then be compared against the Federal Aviation stuff that Beatrice just posted. Bear in mind that confounding variables such as OTC and Rx medications especially opiates and some stimulants (not cocaine), will be need to be accounted for in considering any illicit drug use for those substance classes...but it will depend on how rigorously you want to work the data sets. That said it's about an hours work for a biostatistician if you ask the right questions with your expert (addictions) knowledge. Interesting stuff indeed!! :-)

Sounds like Michael has worked up the "go to" resource for your question. I note the Li et. al. reference which sounds very interesting. 

I checked to see what we have available here in Australia. The ABS and Dpt of Health keep and provide access to data sets using the WHO Drug & Fiream-Induced Related Deaths tabulation guidelines (which you can request access to), but the occupation var has not been made available since 2002, which concurs with what Michael has said about the US.

Thank you all very much for your responses. A huge thanks to you Michael, you confirmed what I have been suspecting.

I have seen a lot of numbers flying around, but none seemed really useful to me. The  positivity figures don't tell the whole picture, neither do the positive tests following accidents.

Substance use is actually only at the surface of where I am going with my question.

Frequent drug use impairs the users beyond the time where they would still test positive. Addicted users in withdrawal could be impaired before they use again. What I am ultimately looking for is to get an understanding of the scale of this impairment, which of course is further complicated by the lack of measures (of impairment) and the differences in how much this impairment actually affects the ability to perform the given task. Coginitve impairment or emotional state will have different consequences for bartenders or pilots. Even if I were to find reliable numbers on frequencies and amounts consumed, any generalized statement on impairment (and risk) would still be just a guess. And when we add drugs to the mix it gets even more complicated. A joint is not a clear unit, the potencies of illicit drugs are mostly unknown and cannot be measured as easliy as "grams of alcohol within t period of time".

Prevalence figures on substance use would be a start. I had not found anything reliable and rather than chiming in and proclaim "its a big problem", I thought I throw my question out there. - you never know...

Dear Michael

Thank you again for yet another enlightening reply.

I completely agree, there are so many confounding factors, and if you include drugs that might not even be considererd "drugs" such as antihistamines, it gets even worse. Zyrtec for example is considered unsafe by the FAA.

My main interest is in drugs that are used primarily for their psychoactive properties. Either for pleasure or because of some kind of psychological distress.

Your alcohol example works well. Although I believe with alcohol we at least can measure it in units, regardless whether it is whisky or beer. With cannabis its a different story. Someone who smokes 10 joints a day with low THC content, will not experience the same impairment as someone who smokes 10 joints of high grade skunk, but they would most likely be in the same group of "multiple times daily. 

And yes, I did order your book this afternoon :-)