Final answer:
Glucagon (GlucaGen) is the drug that a healthcare professional should have available for a patient experiencing an insulin overdose.
Step-by-step explanation:
The drug that a healthcare professional should have available for a patient experiencing an insulin overdose is Glucagon (GlucaGen). Glucagon is a hormone that increases blood glucose levels by stimulating the liver to release stored glucose. In cases of insulin overdose, glucagon can counteract the effects of excess insulin and prevent potentially dangerous hypoglycemia. It is administered as an injectable medication.