We have been examining violence and crime inside the city's shelter system, and now we are taking a look at drug use. In our investigation, we find reports of homeless men smoking synthetic marijuana in bedrooms, and men and woman overdosing from heroin. NY1's Courtney Gross reports the third part of her weeklong series Unsafe Haven.

He is a resident of the 30th Street shelter in Manhattan. With that address, Angel Ortiz has picked up a surprising credential.

"An opiate overdose — you look for signs, the nails may be a purplish-blue because of the lack of oxygen. The person's breathing, uh, if he's breathing," Ortiz said, taking a labored breath as an example.

Ortiz was trained in how to administer a heroin antidote. He has the card to prove it because at the city's homeless shelters, particularly ones for single adults, drugs are inescapable.

The smell of synthetic marijuana wafts through the halls. Overdoses lurk behind bathroom doors.

A review of almost 1,700 critical incident reports in shelters from last year shows that residents are found in possession of substances like 17 rolled cigarettes of synthetic marijuana, also known as K2; bags of crack cocaine; marijuana; a small bag of cocaine; K2 packed inside a Newport cigarette box; crack cocaine and a crack pipe; and heroin.

"Guys who are trying to stay sober, guys who are trying to stay clean and go to work the next day, how debilitating is it for them?" said Mary Brosnahan, head of the Coalition for the Homeless. "It just contributes to that sense of chaos."

According to our review, there were at least 31 overdoses at homeless shelters last year and 25 arrests for drug possession, use, or sale.

"There are substance use challenges in the shelter system that are reflective of substance use challenges in society overall," said Steven Banks, the commissioner of the city's Human Resources Administration.

"We have trained our staff on-site in shelters to be able to use antidotes to deal with potential overdoses," Banks continued.

Within the walls of these so-called sanctuaries, drug addiction has claimed lives.

In October of last year, a woman was found unconscious in the bathroom stall of a Manhattan homeless shelter, her head leaning against the door and a needle nearby. When the stall door was opened, her head hit the ground.

At another Manhattan homeless shelter, a man was found lying on his bed. A sergeant tried spraying a heroin antidote up the man's nose, but it was too late; he was dead.

Sometimes that antidote works. At a Brooklyn shelter, a man was found in the bathroom with a used needle nearby. He was unresponsive and blood was coming out of his mouth. He was given the antidote and he revived.

For the men and women in shelters, these are stories they are used to hearing and telling.

"I mean, the drug problem is prevalent," Ortiz said.

When we asked the de Blasio administration about drugs in shelters, they said they are training staff in how to use this heroin antidote and they are ramping up security.