Firefighters are crediting a timely training class with helping them save the lives of two men who overdosed on heroin. Last month, Braintree firefighters received training on how to use Naloxone nasal spray, commonly known as Narcan, which counteracts the effects of opiates and can revive a person suffering from an overdose. On Feb. 19, just four hours after learning how to use the medication, three firefighters with Engine 3 responded to a 911 call reporting an unconscious man at an Elm Street address, according to a Fire Department press release. The firefighters found a 24-year-old victim lying on the floor and a hypodermic needle in the bathroom sink. The firefighters administered two doses of Naloxone to the man, who regained consciousness by the time an ambulance arrived. Firefighters used Naloxone again on another victim the next day, saving two lives in as many days. “In the past we have had to stand by and do our best for these patients until the arrival of an ambulance,” Fire Department spokesman J. Edward Bulger said in a statement.