It isn't really news that recreational psychoactive drugs should be avoided by those with a history of mental instability, or that persons using psychoactive drugs should seriously reconsider their recreational drug use if they have any contraindications. It has definitely become clearer in the past decade that teenagers should avoid heavy pot usage, as they seem especially susceptible to cannabis-linked psychosis.
Of course, there is the old correlation is not causation thing.
It may be that people with certain psychological traits (and therefore weaknesses) are drawn to psychoactive drugs. Do people who go to certain websites (say geeks to Slashdot) go because of internal factors or because of eating lots of M&M's? Statistically one could show correlation between eating M&M's (or other typical 'sat in front of a computer all day' snack) and reading techie websites.
Anecdotally I feel there is causation, but that it only effects some - just like some people are especially susceptible to contracting lung cancer if they smoke cigarettes, or some people are more susceptible to alcoholism.
Equally of course, whilst it may be true those experiencing contraindications SHOULD stop, it could well be they are in the group least able to do so.
What is untrue is that cannabis is getting stronger to any significant level.
What is true is that moving it from class B to C has made supply easier and heavy use easier.
What is not true is that there has been a large increase in users as a result of moving it from B to C.
What is true is that people being admitted for treatment are more frequently diagnosed as probably needing treatment due to cannabis use.
This leaves us with a drug that has been used for thousands of years, that causes far less damage to non-users than alcohol, and that damages the health of users to a greater or lesser extent than other recreational drugs (including nicotine or alcohol), and that may be especially damaging to some more than others, just like other recreational drugs, and that represents an equal risk overall as some commonly accepted and played sports.
But, of course, viewed like that doesn't make for column inches...