I don't think marijuana is a gateway drug, from what I've seen and heard. People use marijuana for a wide variety of reasons at all ages, and for many people, it simply stops there. I haven't heard of many individual cases or trends suggesting that people move on from marijuana to harder drugs.
Marijuana does not lead to the use of other drugs. People who want to use drugs will find a way to do so, whether or not they use marijuana. Legalizing. especially for medical purposes, will not increase this behavior. In fact, it might keep people from seeking other, more dangerous, illegal drugs because it helps patients that are not being helped by other treatments.
Marijuana is not a gateway drug and numerous scientific studies back this up. The argument that pot is a gateway drug is due to complete and utter ignorance and used only as a narrative to advance a personal belief and objective. These naysayers need to look at the science and make reasoned decisions.
It used to be said that marijuana was a gateway drug, leading users into more dangerous drug use. However, more recently it has been shown that this simply isn't true. Using marijuana does not promote use of harder and more dangerous drugs. Medical marijuana has shown to be very effective, and I think it should be approved in every state.