Her parents were running out of hope. Their teenage daughter, Mary, had been diagnosed with a severe case of obsessive–compulsive disorder (OCD), as well as ADHD. They had dragged her to clinics around the country in an effort to thwart the scary, intrusive thoughts and the repetitive behaviors that Mary felt compelled to perform. Even a litany of psychotropic medications didn’t make much difference. It seemed like nothing could stop the relentless nature of Mary’s disorder.

Their last hope for Mary was Boston-area psychiatrist James Greenblatt. Arriving at his office in Waltham, MA, her parents had only one request: help us help Mary.


Greenblatt started by posing the usual questions about Mary’s background, her childhood, and the onset of her illness. But then he asked a question that no psychiatrist ever had: How was Mary’s gut? Did she suffer digestive upset? Constipation or diarrhea? Acid reflux? Had Mary’s digestion seemed to change at all before or during her illness? Her parents looked at each other. The answer to many of the doctor’s questions was, indeed, “Yes.”

That’s what prompted Greenblatt to take a surprising approach: besides psychotherapy and medication, Greenblatt also prescribed Mary a twice-daily dose of probiotics, the array of helpful bacteria that lives in our gut. The change in Mary was nothing short of miraculous: within six months, her symptoms had greatly diminished. One year after the probiotic prescription, there was no sign that Mary had ever been ill.


Her parents may have been stunned, but to Greenblatt, Mary’s case was an obvious one. An imbalance in the microbes in Mary’s gut was either contributing to, or causing, her mental symptoms. “The gut is really your second brain,” Greenblatt said. “There are more neurons in the GI tract than anywhere else except the brain.”