One of the first studies to use brain scans to examine marijuana’s effects has found that smoking it may be associated with changed brain activation patterns more than a month afterward.

The study may come as a surprise to proponents of marijuana legalization, as it contradicts several previous results showing that there are no significant long-term effects of marijuana use.

Normally human memory formation is associated with a specific pattern in a part of the brain called the hippocampus. The pattern is abnormal in teenagers with a history of marijuana use, Yale University researchers found based on the study.

The marijuana users in the study also performed worse on a simple working-memory tests in which the participants heard a list of words and periodically were asked to repeat a word that was one or two words back on the list. The marijuana users had stopped taking the drug for more than one month before the tests.

Their research, published on June issue of the Annals of the New York Academy of Sciences, involved seven teenagers with a history of marijuana use and 14 without such a history. The authors said the results should be taken with caution: because of the small sample size, the research is merely a “pilot study” and further studies are needed to confirm the findings.

The university’s Leslie Jacobsen and colleagues used functional magnetic resonance imaging in the research, a type of brain scan technology that registers blood flow to functioning areas of the brain. The technique is supposed to reveal which parts of the brain are active at any given time.