They are trying to say that THC can cause depression, but it can take many years. I'm just clarifying (and not downvoting, like the rest of 'hacker' 'news'.. lol) and do not necessarily agree/disagree with OP
long-term "depression" (LTD) is an activity-dependent reduction in the efficacy of neuronal synapses
That doesn't sound like what they were trying to say.
.. then please do elaborate/explain away.... FFS your comment is not at all helpful.