Many people experience a long-lasting effect after having been involved or witnessing violence. "Strong emotions, jitters and sadness or depression may all be part of this normal and temporary reaction to the stress of an overwhelming event."  Many studies have looked at this as temporary - that the effects of violence go away - but a few recent studies showed that the effects can be long-lasting for adolescents and even more so for females.
26 Year Study on the Effects of Violence on Adolescents
A study completed in 2011 evaluated adult health in individuals who had experienced violence during their adolescent years. Surveys were completed in 1974, 1981, 1991, 2000 and 2010 to follow health in thousands of individuals in Sweden. Questions on the survey asked about exposure to violence and the level of violence - for example, participants answered yes or no to whether they were physically injured, whether they were exposed to violence but did not have any injury and whether they were exposed to a situation which was frightening. The surveys also asked questions about health, answering yes or no to whether they experienced symptoms of illness or had been diagnosed with an illness or disease. Participants were also asked to rate their health overall as being "Good," "In between," or "Bad."
After evaluating the responses, researchers found a large difference between how men and women reacted to previous exposure to violence. Women who had been exposed to violence as a teen had increased risk of ill health and rated themselves as being in poor health. There were no substantial health problems in the men who had been exposed to violence during their teen years as opposed to those who had not been exposed to violence.
Long-Term Stress Effects
Another study, completed at Penn State University in 2012, looked at the effect of stress from violent situations on 124 adolescents between the ages of 8 and 13. The participants were from rural and small city communities without a history of exposure to violence in their homes. The participants completed a questionnaire about exposure to violence over the last 12 months. They were then asked to complete a story and were given arithmetic tasks, both common methods of measuring stress levels.
The participants cortisol levels were measured before and after the stress test. Cortisol is a hormone secreted by the adrenal glands. Cortisol levels are usually higher in the morning and lower at night but stress usually results in higher cortisol levels and is responsible for some of the physical responses to stress such as, a burst of energy, heightened memory and lower sensitivity to pain. When we experience the "fight or flight" response to situations, cortisol levels increase. High levels of cortisol over long periods of time has been associated with obesity, high blood pressure, hyerlipidemia and hyperglycemia, which can lead to cardiovascular disease.
According to the results of the study, the males who experienced an increase in violence had decreased levels of cortisol. In other words, as males adapted to violence, their bodies no longer reacted by secreting cortisol. The females in the study did not experience a decrease in cortisol levels. Researchers theorize that females may be able to reduce cortisol levels by talking about the stressful or violent situation. According to Elizabeth Susman, lead author of the study, "If parents and other adults are available to discuss episodes of violence with children, it might help the children, especially females, to reduce cortisol levels." 
"Coping with Stress," Reviewed 2012, July 20, Staff Writer, Centers for Disease Control and Prevention
"Cortisol Connection: Tips on Managing Stress and Weight," Date Unknown, Christine A. Maglione-Garves et al, University of New Mexico
 Exposure to Violence Has Long-term Stress Effects Among Adolescents, 202, July 3, Staff Writer, Penn State University
"Long-term health Consequences of Violence Exposure in Adolescence: A 26-Year Prospective Study," Niclas Olofsson et al, BMC Public Health
"Perceived Stress and Cortisol Levels Predict Speed of Wound Healing in Healthy Male Adults," 2004, July, M. Ebrecht et all, Psychoneuroendocrinology