Final answer:
The most common adverse reaction to dantrolene (Dantrium) is muscle weakness.
Step-by-step explanation:
The most common adverse reaction to dantrolene (Dantrium) is muscle weakness. Dantrolene is a muscle relaxant and weakens muscle strength, which can lead to muscle weakness in patients. This is an anticipated adverse reaction to the medication and should be closely monitored by Nurse Marty.