Popular in the 1970s/80s was the hypothesis that cannabis was a so called gateway drug. There was great concern that trying cannabis could be a gateway that caused users to go seek stronger and stronger highs. Those fears arose from the combination of conditional probabilities (children who use cannabis are much more likely to progress to harder drugs) and sequential order (cannabis usually predates use of harder drugs). But those facts together do not imply causality.
Over the years the validity and relevance of this hypothesis have been challenged[i]. There is now evidence[ii] that suggests that complex interactions among various individual/ predisposing factors and environmental factors (e.g., peer-pressure, family influence, drug availability, opportunities for drug use) drive drug seeking, drug use/abuse, and drug addiction, and these interactions are not necessarily tied to cannabis use alone. It could therefore be suggested that there is a causal path from greater cannabis use to use of hard drugs that is social or psychological, however there is no significant evidence that suggests there is a biochemical link[iii]. The social aspect may simply exist because cannabis is illegal, which forces people to engage with criminals in order to procure cannabis.
The only drug with strong evidence for the gateway drug theory is nicotine, and that is because people sometimes use cannabis and cigarettes together. In fact, instead of the gateway effect, the evidence is stronger that cannabis can reduce the intake of other drugs.
TOP takeaway: There is no significant evidence to suggest that smoking cannabis leads to other drugs. It is suggested that the illicit status of cannabis has had more effect of exposing users, especially young people, to harder drugs.