'Obviously affected' is almost impossible to define in a legal sense. Hence we have an acceptable blood alcohol level of 0.2 here in the UK. No is allowed the luxury of a debate on whether they were impaired or not - exceed that limit on report and you will get dual punishment of instant job loss followed shortly thereafter by a jail sentence. The approach to drugs is even stricter with a zero tolerance policy for any amount whatsoever. There is no discussion about levels of impairment as that is a defence lawyer's opportunity to argue black is white and run rings round an uneducated jury. Hence the rules in all airlines are even simpler than for alcohol - any participation of any kind gets you fired. What I am less certain of is at what point it turns into a court case and inevitable jail.