Final answer:
As of 2014, poisoning due to drug overdoses including heroin and opioid painkillers has become the leading cause of unintentional deaths in the United States.
Step-by-step explanation:
The leading cause of unintentional deaths in the United States has shifted over time. While in 1999, motor vehicle crashes were more than twice as likely to cause accidental deaths compared to drug overdoses, this trend reversed by 2014. At that time, drug overdoses became the leading cause of accidental death, with a nearly 40 percent higher occurrence than car crashes, largely driven by an increase in heroin and opioid painkiller fatalities. Despite the high incidence of drowning, particularly in children aged 12 and younger where it ranks as the second leading cause of death after motor vehicle crashes, it does not lead the statistics overall for unintentional deaths in the U.S. Similarly, while firearms and other causes such as head and traumatic brain injuries, as well as workplace homicides, contribute significantly to mortality rates, they do not represent the top cause of unintentional deaths.
Based on the provided information, the correct answer is poisoning, due to drug overdoses, as the leading cause of unintentional deaths in the United States.