Final answer:
Respiratory depression is the most urgent adverse reaction to report when a client on disulfiram has consumed alcohol, due to the potential life-threatening risks it poses.
Step-by-step explanation:
The adverse reaction that is the priority to report to the provider for a client who has taken disulfiram and consumed alcohol 12 hours ago is respiratory depression. Disulfiram blocks the enzyme aldehyde dehydrogenase, leading to increased levels of acetaldehyde when alcohol is consumed, which causes unpleasant effects such as flushing, nausea, dizziness, and sweating. However, the most urgent concern in this context is respiratory depression, which can be life-threatening and requires immediate medical attention.