Scientists at the Centre for Addiction and Mental Health in Toronto have figured out why antidepressant medications don’t help half the people to whom they’re prescribed to deal with clinical depression. Today’s antidepressants, they say, don’t affect a protein in the brain that reaches high levels when patients are depressed. It’s called monoamine oxidase A or MAO-A. At very high levels, it actually breaks down feel-good chemicals such as serotonin in the brain. Rather than reversing MAO-A’s damaging habit of removing these all-important chemicals from the brain, current antidepressant drugs just raise serotonin levels. High levels of MAO-A also make people more susceptible to future depression. The researchers say understanding the biochemical processes that contribute to depression are vital to finding an effective, long-lasting remedy—and they predict that future antidepressants will tell the brain to produce less MAO-A.