I've got to say it concerns me how many people try to justify that using drugs is ok. As for alcohol, it may well be legal. But, the same rules apply.
If you have consumed enough to affect you at work be it one or even two days before then don't be at work.
If you agree to a contract of employment then there are no excuses, you either adhere to it or find another job, preferably behind a desk where you don't make any decisions.
Yes in the 1960s when dinosaurs roamed the earth people did drink and drive, sup 4 pints at lunch time. But culture has evolved.
In a professional environment there is no place for alcohol or drugs.
If you think you are entitled to do as you wish in your own time you are wrong. You have agreed to a contract which requires that you do not attend work under the influence of drugs or alcohol. So, at home, in your own time you must ensure that when you go to work there are no prohibited substances in your system.
The second you attend work with traceable levels of illegal substances or levels above what is specified in your contract by your employer (this can be lower than the legal driving limit) you are committing gross miss conduct and if for me will no longer be employed. Others may be more lenient but don't get your hopes up.
If its drugs you would also be reported to the police.
We work with lethal machinery, in high risk environments. I fail to see why you wouldn't want to be of full awareness when at work.
The "you drink, that's no different to taking drugs" argument is child like. Gets nobody any where, it brings no validity to the debate. The use of alcohol is legal. But to be under its influence at work makes you equally sackable at work as the bloke who's had a joint the night before, or vice versa, any other drugs etc.
Even if you have no contract, self employed bla bla bla, with the equipment we use you have a legal obligation to be drug and alcohol free.