Addiction - misconduct or incapability?
If an employee commits overt misconduct in the course of his/her addiction, e.g. theft to pay for drugs, then that is clearly misconduct regardless of its cause. Nothing in its being a direct consequence of the addiction requires you to put up with this. But what about the underlying problem?
Especially if you have published rules prohibiting the possession and/or consumption of drugs or alcohol at work, it's tempting to treat this matter also as one of misconduct rather than illness. There is no rule of law that employers have to help those on their books struggling with drug or alcohol addiction, no absolute duty to make reasonable adjustments to accommodate their problems, and no obligation to tolerate sub-standard work or behaviours arising as a result.
Despite that, ACAS and the Employment Tribunals encourage employers to treat addiction as an illness rather than a conduct question (though it's recognised that this is far easier in respect of alcohol than proscribed drugs, where the employee’s use or possession and/or an employer’s inaction could itself constitute a criminal offence).
Without overt misconduct, therefore, your best option is to seek to treat the addiction in the first instance as a capability question. You can then move to misconduct if the employee does not admit the problem or take the steps you reasonably require to address it, e.g. attending courses, taking medication, staying off the substances, etc.