Could he be signed off with stress, then use a private agency to deal with his addiction.
A few issues come to mind, not knowing his capacity in the NHS.
Access to the drugs trolley, oh Mrs Smith had her dose of oramorph, tick on computer, but she actually didn't DH had her dose. Mrs Smith queries 2 hours later, can I have anything stronger, the pain has got quite severe. You had a dose of oramorph 2 hours ago, I'm afraid that's one of the strongest meds we can give unless you're having say a heart attack. I remember having codeine, but no oramorph. Well it states you had 5ml of oramorph here, so nothing for another 2 hours.
Then there's access to drugs or the drugs trolley.
Impaired fitness due to intoxication / withdrawal.
I think it's very risky working in either state.
Most towns / cities have services for alcoholics / substance abusers.
I guess the critical thing is, is your husband using due to pain, or due to addiction. There's dependency that occurs when you've been on a drug a long time. This isn't the same as addiction, where you are taking something for a high or other reward.
You need to discuss this seriously with him, as if it impairs his fitness to practise, he shouldn't be working. I've had nurses and paramedics deliver IV morphine. So if you're on the cusp of withdrawal, you're in a pretty negative state where desperation starts kicking in.
You want to believe the person wouldn't jeopardise their position, but with addiction it's very tricky.
I've been in situations where I was unable to ingest medications due to vomiting. Withdrawal is a total beast. You quite simply want to die or for it to stop.