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Smoking marijuana can significantly increase a person's risk of depression and anxiety, according to a new study.

New research reveals that people who use marijuana have brains that are less capable of responding to dopamine, the neurotransmitter that triggers motivation for seeking reward.

American researchers said that the latest findings support previous studies suggesting that the use of marijuana can leave people feeling inhibited, lazy and indifferent.

The latest study involved 48 people and used the stimulant drug Ritalin to understand how marijuana influenced the brain. Researchers explain that the ADHD drug was used in the study because it raises levels of dopamine the brain.

The findings revealed that marijuana users showed significantly lower dopamine responses compared to healthy controls, according to personality tests and brain scans.

Researchers explain that lower dopamine levels could be partly responsible for drug craving, depression and anxiety- all of which have been linked to marijuana addiction.

"Marijuana abusers show lower positive and higher negative emotionality scores than controls, which is consistent, on one hand, with lower reward sensitivity and motivation and, on the other hand, with increased stress reactivity and irritability," researchers wrote in the study.

"We found that marijuana abusers display attenuated dopamine (DA) responses to MP, including reduced decreases in striatal distribution volumes. These deficits cannot be unambiguously ascribed to reduced DA release (because decreases in nondisplaceable binding potential were not blunted) but could reflect a downstream postsynaptic effect that in the ventral striatum (brain reward region) might contribute to marijuana's negative emotionality and addictive behaviors," they added.

"Moves to legalize marijuana highlight the urgency to investigate effects of chronic marijuana in the human brain," researchers concluded.

The findings are published in the journal Proceedings of the National Academy of Sciences of the United States of America.