WE know that occasionally people react badly to marijuana. Some withdraw into anxious, glassy silence. Some responses are more horrifying. In a recent, much discussed case, a Denver man bought cannabis-infused Karma Kandy and hours later — perhaps after also taking prescription pain medication — began raving about the end of the world and killed his wife.

Marijuana is more dangerous than many of us once thought. For one thing, cannabis use is associated with schizophrenia, an often devastating disorder in which people can hear disembodied voices that sneer, hiss and command. A 1987 study published in The Lancet, the London-based medical journal, followed more than 45,000 Swedish military conscripts. Those who said on a conscription questionnaire that they had used cannabis more than 50 times were six times as likely, 15 years later, to have been diagnosed with schizophrenia than those who said they had not used it. There have been many more research papers since. A 2007 meta-study, also published in The Lancet, examined a series of them and concluded that there was a consistent increase in the incidence of psychosis — the radical disconnect from reality characteristic of schizophrenia — among people who smoked marijuana, with most studies showing a 50 to 200 percent increased risk among the heaviest users.

The causal arrow is complicated here. This does not prove that marijuana brings on schizophrenia. It could be that people with incipient schizophrenia are drawn to cannabis. But it is clear that cannabis can lead to passing paranoid and hallucinatory experiences, and a 2014 psychiatric overview argued that cannabis could not only cause those symptoms to persist, but to develop into a condition that looks like schizophrenia. Jim van Os, a leading European schizophrenia researcher, suggested that marijuana might be responsible for as many as one in seven or eight cases of schizophrenia in the Netherlands.

To be sure, that increased risk is pretty low: About one in 100 people will develop schizophrenia. The unnerving question is whether in this country, with its history of gun violence and its easy access to guns, a person with a paranoid reaction is more likely to act violently.