Across the military, the precise dimensions of the problem are elusive, especially since the different branches largely keep their own statistics. Many studies do not distinguish between service members who have seen battle and those who have not. What is more, behavior becomes far harder to track when service members leave the military.

Even so, a variety of surveys, as well as anecdotal evidence and rising alarm in many military communities, indicate growing substance abuse among recent combat veterans. Of particular concern are members of the National Guard and reserves, as well as recently discharged service members, who can lose their bearings outside the camaraderie and structure of the military.

In the Army, which has the bulk of the troops in Iraq and Afghanistan, the Pentagon’s most recent survey of health-related behavior, conducted in 2005 but released last year, found that for the first time in more than 20 years, roughly a quarter of soldiers surveyed considered themselves regular heavy drinkers — defined as having five or more drinks at least once a week. The report called the increase — to 24.5 percent in 2005, from 17.2 percent in 1998 — “an issue of concern.”

Perhaps the best monitor of recent combat veterans’ mental health is the Pentagon’s postdeployment survey. Reflecting concern about heavy drinking, the latest report, published last November, introduced a question about drinking habits. Of the 88,235 soldiers surveyed in 2005 and 2006, three to six months after returning from war, 12 percent of active-duty troops and 15 percent of reservists acknowledged having problems with alcohol.

While drug use decreased substantially after 1980, when the military cracked down, it has increased slightly in the Army and the Marines since 2002, the behavioral survey said. Experts say that, in some cases, troubled combat veterans are more prone to use drugs after leaving the military.

In general, studies find that drinking is more prevalent in the military than in the civilian population; the behavioral survey reported that heavy drinking among 18- to 25-year-old men in the Army and the Marines was almost twice as common as among their civilian counterparts.

Heavy drinking or drug use frequently figures in what law enforcement officials and commanders at military bases across the country say is a rising number of crimes and other examples of misconduct involving soldiers, marines and recent veterans.