Someone who wants to commit suicide, or a homicide, is not prevented by a lack of guns, but access makes it easier. 

A meta-analysis, a statistical look at various studies that shows patterns, published in Annals of Internal Medicine, pooled results from 15 investigations, slightly more than half of which were done after a 1996 federal law prohibited the U.S. Department of Health and Human Services from funding research that promotes gun control.

The authors determined that someone with access to firearms is three times more likely to commit suicide and nearly twice as likely to be the victim of a homicide as someone who does not have access. Their review excluded studies that relied on survey data to estimate gun ownership and focused instead on studies that included more specific information about whether victims had access to guns.

All but two of the studies were done in the United States. The review included studies about deaths by suicide and homicide but not accidental deaths.

There were gender differences related to gun use in homicide and suicide. Men were nearly four times more likely to commit suicide using a gun than when firearms were not accessible (which makes sense, they couldn't use a gun they don't have access to), while women were almost three times more likely to be victims of homicide. 

The authors did not control for other sociological factors, like how many women were involved with or engaged in criminal activity. They do seem to suggest that not having guns would reduce suicide, which has not been found to be the case in countries where guns are banned. 

"Our analysis shows that having access to firearms is a significant risk factor for men committing suicide and for women being victims of homicide," said Andrew Anglemyer, PhD, MPH of U.C. San Francisco. "Since empirical data suggest that most victims of homicide know their assailants, the higher risk for women strongly indicates domestic violence."

Firearms, overwhelmingly handguns, are involved in over half of US suicides and two-thirds of homicides, according to 2009 data from the 16-state National Violent Death Reporting System, which is run by the U.S. Centers for Disease Control and Prevention. About 75 percent of suicides occur in the victims' homes, and a similar percentage of female homicide victims die in their homes. The figure is about 45 percent for men.

Since not all of the studies assessed whether victims had firearms in their homes, the meta-analysis does not draw conclusions about the associations between suicide or homicide and the location of the firearms, but merely whether victims had access to them.

Researchers adjusted for biases they say they detected in the original studies, such as failing to account for mental illness, domestic violence or arrest history or inadvertently influencing the reports of victims' friends and relatives about whether they had access to firearms. In some cases, such as in the selection of participants for studies on suicide, the bias in the original studies may have underestimated the association between access to firearms and suicide, because both study and comparison groups were recruited from health care settings where they may have been seeking treatment for suicidal planning.

Of the 15 studies included in the meta-analysis, the only one that did not find a statistically meaningful increase in the odds of death associated with access to firearms was from New Zealand, where guns are much less available than they are in the United States. And even that study did find an increase, although not a statistically significant one.

Studies have increasingly shown that murders, shooting sprees and suicides have a substantial mental health correlation and there have been calls to restrict gun ownership by people with a history of mental illness, or who are on medication for mental issues.