As marijuana legalization spreads across the country, people are consuming more of the drug, more often and at ever-higher potencies. Most of the tens of millions of people using marijuana, for health benefits or for fun, don’t experience problems. But a growing number, mainly heavy users, have experienced addiction, psychosis and other harmful effects, The New York Times found.

“Cannabis is a lot of things at once,” said Dr. Kevin Gray, a psychiatrist and specialist in bio-behavioral medicine at Medical University of South Carolina Health. “It can be medically therapeutic. It also can be highly problematic.”

In interviews and surveys, hundreds of people told The Times about serious — sometimes frightening — symptoms that they were stunned to learn could be caused by cannabis. Here are some of their stories.

Cannabinoid Hyperemesis Syndrome

‘A Real Danger’

David Krumholtz, an actor known for films like “10 Things I Hate About You” and TV shows like “Numb3rs,” resumed smoking marijuana in 2016, after a decade-long break. Within months, he started to experience cycles of intense nausea and vomiting — a sometimes debilitating condition called cannabinoid hyperemesis syndrome. It can lead to dehydration, seizures, kidney failure, cardiac arrest and even death in rare instances.

He lost 100 pounds and was in and out of emergency departments. At home in New Jersey, he would spend 10 hours at a time in hot baths, which for unknown reasons can temporarily relieve symptoms.