Scientists do not yet know how the use of marijuana relates to mental health problem. Among the difficulties in this kind of research are determining whether marijuana use precedes or follows mental health problems; whether one causes the other; and/or whether both are due to other factors such as genetics or environmental conditions. High doses of marijuana can induce psychosis (disturbed perceptions and thoughts), and marijuana use can worsen psychotic symptoms in people who have schizophrenia. There is also evidence of increased rates of depression, anxiety, and suicidal thinking in chronic marijuana users. However, it is not yet clear whether marijuana is being used in an attempt to self-medicate an already present but otherwise untreated mental health problem, or whether marijuana use leads to mental disorders (or both).

When a nursing mother uses marijuana, some of the marijuana's THC is passed to the baby in her breast milk. This is a matter for concern, since the marijuana's THC in the mother's milk is much more concentrated than that in the mother's blood. One study has shown that the use of marijuana by a mother during the first month of breastfeeding can impair the infant's motor development (control of muscle movement). This work has not been replicated, although similar anecdotal reports exist. Further research is needed to determine whether marijuana's THC transmitted in breast milk has harmful effects on development.

Doctors advise pregnant women not to use any drugs because they might harm the growing fetus. Although one animal study has linked marijuana smoking to loss of the fetus very early in pregnancy, two studies in humans found no association between marijuana smoking and early pregnancy loss. More research is necessary to fully understand the effects of marijuana smoking on pregnancy outcomes.

Some scientific studies have found that babies born to women who used marijuana during their pregnancy display altered responses to visual stimulation, increased tremors, and a high pitched cry, which may indicate problems with nervous system development. During pre- and early school years, marijuana-exposed children have been reported to have more behavioral problems and difficulties with sustained attention and memory than nonexposed children.

Researchers are not certain whether any effects of marijuana smoking during pregnancy persist as the child grows up; however, because some parts of the brain continue to develop into adolescence, it is also possible that certain kinds of problems will become more evident as the child matures.