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Teenagers who use marijuana heavily grow up to have poor memories and also have brain abnormalities, a new study shows.

The study cannot say which came first — the brain structure differences or the pot use. But it suggests there could be long-term effects of heavy marijuana use.

A team at Northwestern University looked at 97 volunteers with and without mental illness. The dope smokers said they’d used marijuana daily starting at age 16 or 17, and said they had not used other drugs.

"The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems."

The daily marijuana users had an abnormally shaped hippocampus and performed about 18 percent more poorly on long-term memory tasks, the researchers reported in the journal Hippocampus.

The hippocampus is a part of the brain used in storing long-term memory.

"The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems and to sustain our relationships with friends and family," said Dr. John Csernansky, who worked on the study.

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Previous research by the same Northwestern team showed heavy pot smokers had poor short-term and working memory and abnormally shaped brain structures including the striatum, globus pallidus and thalamus.

"It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse," Matthew Smith, who led the study, said in a statement.

"But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause."

The effects lasted into the early 20s, even after the volunteers stopped using marijuana, the researchers said.

"Advanced brain mapping tools allowed us to examine detailed and sometimes subtle changes in small brain structures, including the hippocampus," said Lei Wang an assistant professor of psychiatry who also worked on the study.

In addition, the team found that patients with schizophrenia who smoked marijuana did about 26 percent more poorly on memory tests than other schizophrenia patients.

Four states and the District of Columbia have legalized it for recreational use.

Twenty three states plus Washington D.C. have legalized cannabis for medical use. This week, a group of U.S. senators introduced a bill that would bar the federal government from prosecuting medical marijuana use in states where it's legal.

"It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse."

Other studies have found that marijuana is the safest substance to abuse — far less likely to kill you than alcohol, tobacco or drugs such as heroin.

Marijuana's been found to help some medical conditions, such as cancer pain and nausea. But the American Academy of Pediatrics opposes non-medical use of marijuana by kids.