Of course they won’t all progress, but almost 100% of heroin users started on it.
There is a frequent pattern of drug use: —
Beer/wine—>cigarettes/spirits—>marjjuana—> other illegal drugs.
Professor Denise Kandel and others of the Dept. of Psychiatry and School of Public
Health, Columbia University, New York, have found this progression in most of their
investigations, published in various journals from the seventies till the present, including,
Science, The American Journal of Public Heal/h, Journal of Drug Issues.

We must try to stop our children from smoking in the first place. Smokers have already mastered the required technique.
A MORI poll in 1991 found that 50% of smokers had tried an illegal drug compared to only 2% of non-smokers.

The more they use, the greater the risk of pro gression.
Of those who had used cannabis more than 1000 times, 90% went onto use other drugs.
Between 100 and 1000 times, it was 79%, dropping to 51% between 10 and 100 times.
Even 1 to 9 times saw 16% go down the slippery slope whereas only 6% of non-users
succumbed (Denise Kandel, 1986)

Use of cannabis and cocaine have been linked in several studies.
“The linkage between marijuana use and later heroin or cocaine use is 10 times greater than the evidence of linkage between cigarette smoking and lung cancer”, Clayton & Voss, US Journal of Drug & Alcohol Dependence, 1982. And from Science, 1997. ‘22% of Dutch youths over 12 who have ever used cannabis, have also used cocaine’. ‘The main predictor of crack cocaine use is previous daily use of pot’ (PRIDE, USA)

Recent research from Australia and New Zealand confirms all this.
Youngsters smoking one joint a week are 60 times more likely to progress, the strongest association is among 14-15 year olds (Fergusson and Horwood, Addiction 2000). Genetics was ruled out when a study of 300 pairs of same-sex twins discovered those using cannabis before the age of 17, were 2-5 times more likely to have drug problems and dependency later in life, than their non-using siblings (Linskey etal, JAMA 2003)

Some users say they are looking for a bigger and better ‘high’,
 rather like small children jumping from a higher step or swinging ever higher.
When people become tolerant of a ‘high’, they seek a more potent drug, or the withdrawal symptoms from one can be alleviated by use of another (H. Ashton, Current Opinion in Psychiatry, 2002)

Drug dealers are often blamed for pushing other drugs.
The Dutch ‘experiment’ was supposed to separate the markets of ‘soft’ and ‘hard’ drugs. This has not stopped the progression. Holland has twice as many heroin addicts per capita than Britain (Trimbos Institute 1997; Schwartz R, Hospital Practice May 1991)