At what level of use? And for which demographic groups?
Some of the effects, notably the precipitation of psychotic disorder (related to schizophrenia), act on a developing brain but not a mature brain. Thus, this effect is a danger for users under the age of 25 years old.
Certainly product warning labels would be appropriate - perhaps even a higher minimum age for marijuana purchase, although I have a sneaking suspicion that alcohol is also not good for developing brains.
Other effects, such as the loss of initiative and motivation, seem to affect older people as well as younger people. This is because some of the components of marijuana cause the death of certain cell types which are crucial for these functions.
So research has established that this is the causal mechanism?
The deficits in motivation and initiative were observed three months after the last use of marijuana. Thus, the effect can have a long duration, but this study did not establish whether the effect eventually dissipates and if so, after how long.
Slim support for claims of "long-lasting, if not permanent."
Alcohol is less of an issue because it lasts for such a short while in the body. For evolutionary reasons, there are at least two detoxification pathways to destroy alcohol. The active compounds in marijuana, on the other hand, are lipid soluble, so they partition into fat tissues, and there is no dedicated metabolic pathway for destroying exogenous cannaboids. In general, the longer a substance stays in the body, the more harmful it is.
So research has established that this is the causal mechanism?
The brain damage to these structures has been documented through functional imaging and other means.
Slim support for claims of "long-lasting, if not permanent."
The fact that the effects are still observed after three months means that they are long-lasting. The study did not do later follow-ups to determine if those effects are permanent. They could be permanent, since the brain typically does not regenerate.