Define healthy
"The UK Advisory Council on the Misuse of Drugs (ACMD) reviewed the evidence in depth and concluded (2002, p. 8) “... no clear causal link [between prevalence of cannabis consumption and schizophrenia] has been demonstrated.”"

That relates to not-already-vulnerable people, however. It's suggested that heavy cannabis use in those with a family history of mental illness can indeed precipitate the onset of psychosis.
(tty, Sun 19 Sep 2010, 20:55, 1 reply)

Also, there are many other factors besides genetics that can influence schizophrenia - not least the behaviour of the people in the sufferer's daily life.
It's such a complex issue that in general you cannot reasonably stand up and say "yes, this person's condition has been caused by [X] and [X] alone."
( The Mock TurtIe ™ --- Thinks you are a cunt, on, Sun 19 Sep 2010, 21:22, closed)