So there is no protection against discrimination?
Sorry about that. It is sad. Being jobless is sad.
But discrimination? I suppose that in the broader meaning of the word what you have unfortunately experienced can be seen as discriminatory. But the high risk of relapse and repeated recurrences are well known. It isn't a well kept secret. And if the employer has a choice, well . . . .
Anyway a history of drug abuse is not classified in the same category as race, age, gender, religion, national origin, physical or mental disability. Each of which are human conditions and (sometimes) proper subjects of forbidden discrimination in the work place.
Don't give up. Don't disclose unless absolutely necessary. And don't relapse! Every day drug free can mean another or more at the end.