Iran does not keep official statistics on marijuana use. But anecdotal evidence and figures from rehabilitation clinics indicate that pot smoking is widespread in Iranian cities. Hossein Katbaei, the director of one such clinic, Camp Jordan, said the number of patients his staff was treating for marijuana abuse had quadrupled over the last five years.

Mr. Katbaei, a former truck driver with a long ponytail, and other addiction experts say young Iranians often become caught up in a vicious cycle. With widespread unemployment and forbiddingly high house prices, many young adults are forced to live at home, leading to lives of isolation and depression that they seek to escape through marijuana.

Marijuana is internationally often viewed as a nonaddictive drug. But those using it frequently can become dependent on it. According to the United States’ National Institute of Drug Abuse, teenagers using marijuana are four to seven times more likely than adults to develop a marijuana disorders. In severe cases, the institute says, this can lead to addiction.

Iranian experts point out that a growing percentage of marijuana produced inside the country is laced with other drugs. Also, most seeds are smuggled in from Amsterdam, and many are genetically enhanced to produce more strength.

With the rise in marijuana use, the patients in Mr. Katbaei’s clinic have changed.

“They are from middle-class families, often reasonably well off,” said Youssef Najafi, a former drug addict who is now a counselor at the clinic. “They feel useless. Live at home. Their future is one big unknown. Some years ago we would only have a couple. At first they think it is harmless, but those who use it too much get depressed and ultimately psychotic.”

Few older Iranians, whether health officials or parents, know much about marijuana or its effects, Mr. Najafi said. There is no government effort to inform people about the effects of marijuana use. In 2013, the current head of the police, Ali Moayedi, told state media that marijuana did not exist in Iran.

But during a counseling session at the camp, marijuana was very much on the top of the list of what most patients had been using while they were out. “Here in Iran at least, marijuana is really a gateway drug,” Mr. Najafi said