The correlation is that those who use "hard" drugs have often also previously used other drugs, including alcohol and prescription medication. Flipping that around to say the using cannabis leads to injecting heroin is the classic post hoc ergo propter hoc logical fallacy.
Perhaps the correlation is a result of cannabis being illegal in your country: if you break one law (cannabis use), the barriers are down and you may as well be hanged for a sheep as a lamb and explore stronger remedies for your troubles.
So the questions that need to be investigated are these. Is there in fact any kind of correlation? If there is a correlation, is it causative? Of course, no true believer needs these questions answered, which is why the research into these things in not legal in countries where prohibition enforcement and punishment is big industry.
Perhaps the correlation is a result of cannabis being illegal in your country: if you break one law (cannabis use), the barriers are down and you may as well be hanged for a sheep as a lamb and explore stronger remedies for your troubles.
So the questions that need to be investigated are these. Is there in fact any kind of correlation? If there is a correlation, is it causative? Of course, no true believer needs these questions answered, which is why the research into these things in not legal in countries where prohibition enforcement and punishment is big industry.