Most mass shootings caused by untreated mental illness?

Letter submitted to the Globe and Mail:

I challenge Andre Picard to support his claim that, "The vast majority of shooters have also suffered from untreated mental illness". This is an unbelievable remark given that it is well known that many drugs for psychological conditions (such as the SSRI antidepressants) can cause uncharacteristic outbursts of violent behaviour. See Dr. David Healy's "Let them eat Prozac", for example. Even in cases where shooters have recently stopped their medications, it does not remove responsibility from the medications since it is known that many psychological drugs have withdrawal symptoms which can exacerbate the side effects of the drugs for some time after stopping. Healy pointed out that drug companies know this and use it to their advantage since study subjects going from drug A to B will suffer less in the short term than study subjects going from drug A to placebo. Andre Picard should not be intimidated by the clout of pharmaceutical companies and should investigate how many of these shootings involve mental illness being treated, never treated or recently halted. Then we can have an adult debate over the role of both behaviour abnormalities and the drugs often recommended to treat these conditions.