When Mark Horowitz was 21, he began taking antidepressants. 

At the time, he was feeling a "bit miserable" in school — like "a neurotic, Woody Allen kind of guy." His medical provider suggested he start taking a selective serotonin reuptake inhibitor, or SSRI.

He never imagined the debilitating withdrawal symptoms he'd experience 13 years later while trying to come off the drug. 

Horowitz recalled waking up in "animal terror," feeling like he'd just been chased off a cliff, or enduring a "state of panic" so pervasive he took to running several miles a day until his feet bled. For the first time in his life, he thought about killing himself.