More than expected? Assessing departures from additivity in health care utilization after mild traumatic brain injury in military personnel with pre-existing mental health conditions.

Studies of the interaction of two diagnoses on subsequent health-care utilization should examine both additive and multiplicative interactions. The greater-than-additive findings in this study indicate that there may be synergy, for at least some patients, between mTBI injury and mental health that complicates the treatment course.