Emily, a straight-A and B sophomore, developed persistent migraines about two weeks before she wound up in the ICU early on Dec. 8, said Bryant. One bad migraine even sent her to the ER, and doctors scheduled an MRI.

But anxiety and claustrophobia prevented Emily from getting the test.

Bryant said doctors at Memorial Hermann-Texas Medical Center said the migraines were possibly related to using the drug.

"We correlated the time she got migraines with the time she started smoking this stuff," he said. "In their professional opinion, they think it's related. But medically speaking, they don't have a picture of her brain from before and after, so they can't say."

While her family doesn't know how long she'd been using the drug, her stepfather suspected she started around two weeks before the night that sent her to the hospital.

Common side effects to smoking synthetic marijuana include bloodshot eyes, disturbed perceptions and a change in mood, said Dr. Melinda Campopiano, a medical officer with the Substance Abuse and Mental Health Services Administration.

"People can become very agitated or can be come unresponsive -- conscious but not reacting normal to situations," she said. They may also appear paranoid or describe hallucinations. Some of the more potentially serious effects include an elevated heart rate and elevated blood pressure.

Campopiano said she had never heard of a patient having a stroke in these circumstances, but she described how high blood pressure could lead to one.

"Generally, strokes are caused by restricted circulation, or a blood clot that blocks circulation. What we would be looking at with Spice, or K2, is the restrictive circulation model," she said.

Bryant told CNN that doctors diagnosed his daughter with vasculitis, which is an inflammation of the blood vessels. The vessels going into Emily's brain were constricting, limiting blood and oxygen flow. Campopiano confirmed that vasculitis is one of the causes of strokes of this type.

"One of the difficulties is that there's no existing toxicology screen that can reliably detect these substances," said the physician. "There could very well be harms out there that we don't know about yet."

'She was literally just a shell'

Emily complained of a migraine and took a nap at her house after allegedly smoking Spice with friends on December 7, said Bryant. She woke up a different person.

Stumbling and slurring her words, she morphed into a psychotic state of hallucinations and violent outbursts, her family said.

They called 911 after they realized she had "done something," some drug, said her stepfather. The Harris County Sheriff's Office confirmed they visited the house but declined to provide details.

When paramedics arrived, they restrained her and rushed her to a Houston-area hospital, where she was admitted to the ICU.

She bit guardrails and attempted to bite those trying to help her. Hospital staff strapped Emily down in the bed, said her sister.

"We thought once she comes down off the drug, we'd take her home and show her the dangers of this drug," said the 22-year-old. "We didn't think it was as big of a deal until 24 hours later she was still violent and hurting herself. We realized you're not supposed to stay high this long."

To keep Emily safe, doctors put her in an induced coma.

After days in the sedated state, an MRI revealed she had suffered several severe strokes, said Bryant.