Not defending drug use at work, but if you're to be fired it should be for the performance related issues you mentioned caused by the drug, not because of the drug itself.
Edit for more context:
Depending on the drug, substances consumed outside of working hours would still show up in a drug test. Firing someone for something they do outside of work shouldn't be acceptable.
If you can prove they consumed drugs that would impair their ability to perform their job during working hours, that'd be a valid cause for dismissal. Drug tests don't measure that precisely enough though.
Not to mention, drug tests are not infallible. They can come back positive for morphine if you eat a poppy seed bagel in the morning [1].
Edit for more context:
Depending on the drug, substances consumed outside of working hours would still show up in a drug test. Firing someone for something they do outside of work shouldn't be acceptable.
If you can prove they consumed drugs that would impair their ability to perform their job during working hours, that'd be a valid cause for dismissal. Drug tests don't measure that precisely enough though.
Not to mention, drug tests are not infallible. They can come back positive for morphine if you eat a poppy seed bagel in the morning [1].
[1] https://www.snopes.com/fact-check/poppy-seeds-alter-drug-tes...