This fascinating Illinois State Bar Ethics Opinion 17-01 (click through for link) presents this question under the state’s RPC 1.6(c)which mandates disclosure “to the extent the lawyer reasonably believes necessary to prevent death or substantial bodily harm”. :
The inquiring attorney has a client who is addicted to heroin and opioids, and also takes cocaine, marijuana and methadone. The client is arrested for possession of a controlled substance, and appears severely impaired during court hearings, but remains silent before the Judge, allowing the attorney to do the speaking. The client is unable to stop consuming heroin and continues to be in violation of bond conditions.
In essence the bar committee finds insufficient immediacy of harm but suggests that in appropriate circumstances RPC 1.14 Diminished Capacity may allow the lawyer the ability to take protective steps.
– GWC
Professional Responsibility: A Contemporary Approach 