I'm sure we've all heard this line at least once in our day: "If you use marijuana, you'll move on to harder drugs." This is often a person's only line of defense when arguing about the negative effects of cannabis. The fact is, there is nothing in cannabis that makes people want to try a more dangerous substance. Many users however do move on to harder things but it is completely matter of individuality.
One of the reasons so many people believe this theory is that marijuana is the most widely used illicit drug, so it's usually the first one that a person comes in contact with. It has actually been statistically proven that most teens experiment with tobacco and alcohol before ever trying marijuana. However since they are not completely illegal substances, many fail to recognize them as drugs and look past them to cannabis.
The fact that marijuana is an illegal drug may also play a role in its users moving on to heavier narcotics. For instance, a person might refuse to try marijuana because it's prohibited but proceed to drink themselves to death with very dangerous and very legal alcohol. Marijuana users would also be less likely to come in contact with people selling more dangerous substances if cannabis was legal. Some dealers who distribute marijuana also sell other drugs which they might offer to the buyer.
Several studies on the gateway theory have taken place over the past few years, none of them have concluded that marijuana leads to heavier drugs. They conclude that most marijuana users do not graduate to harder substances at all. They do however state that most hard drug users did start with marijuana. There are links below for results and statistics.
I'm not trying to imply that that no one that uses marijuana will want to take the next step. Some will see it as a mind opening experience that makes them curious about other chemicals. That is caused by their personality, genetics, and personal experiences. If someone has an addictive personality, they're more likely to try something else. People who use marijuana are still capable of making informed and rational decisions.
This theory can never be proven because there is no scientific evidence to do so. It has been debated for decades and will probably remain in peoples' arguments for many more.
Here are some links to studies and articles on this subject:
Cannabis Research
Drugscience.org
Drug War Facts
RAND Corporation
