Cannabis

It's the very fact that cannabis is illegal that may be the strongest factor in associations between heroin use and cannabis use. By forcing the user to criminalise him or herself in order to get supplies of cannabis, the system also forces the user to come into contact with a culture in which other drugs, especially Ecstasy and Acid, but also coke and smack, are being traded. If you hang around with people who smoke pot, chances are you're also going to be hanging around with people who drop pills and maybe snort coke. Separate the two out - by, for instance, having tolerated outlets for selling weed, as in Amsterdam, and you break this connection and maybe make it less likely that cannabis will act as a 'gateway'.

Fact is, there are millions of cannabis smokers out there who have never touched anything harder.
No one has ever died from an overdose of cannabis. The amount needed to kill you compared to the amount needed to produce active effects (i.e., get you high) is something like 10,000 times as much, maybe more. You certainly get sick if you've had too much, and there can be psychological effects, but physically, cannabis is a very safe drug. Compare this to alcohol, where the ratio of fatal dose to active dose can be as low as 10:1 in some cases. The long-term effects of alcohol use are also more damaging. Alcohol-related deaths run into the tens of thousands every year in Britain alone, and this does not include people killed by drunken drivers, beaten up by pissed-up rednecks (or husbands), etc. etc. Cannabis, on the other hand, reduces violent tendencies. If both cannabis and alcohol were illegal but there was a choice of legalizing one, I know which one I'd choose.