Unfortunately, that question isn't possible to answer without qualification, because when you say “dangerous”, are you talking about acute danger? Are you talking about fatality? Are you talking about serious impairment of function in the immediate period of action of the drug, or are you talking about long-term health problems? The answer is different for different drugs, according to which type of danger you are talking about.
On May 1st, 2014. See this statement in context.