Teenage marijuana use and Schizophrenia

Your parents are not full of it.  They have reason to be concerned.  It is not clear cut that your marijuana (cannabis) use will cause you to develop schizophrenia, although the fact that your brother is diagnosed with it does increase the risk.  It may already have caused other changes in your brain, however.

I have worked with a lot with people who have schizophrenia.  But I am not an expert on the causal relationship between marijuana use and development of schizophrenia or other psychotic conditions. When in doubt, I check Wikipedia first:

http://en.wikipedia.org/wiki/Marijuana#Long-term_effects

"... effects [of cannibis use] on intelligence, memory, respiratory functions and the possible relationship of cannabis use to mental disorders such as schizophrenia, psychosis, depersonalization disorder and depression are still under discussion."

"A 35-year study published August 2012 ... provides objective evidence that, at least for adolescents, marijuana is harmful to the brain. It was found that the persistent, dependent use of marijuana before age 18 showed lasting harm to a person's intelligence, attention and memory. Quitting cannabis did not appear to reverse the loss."

http://en.wikipedia.org/wiki/Schizophrenia#Environment

"The greatest risk for developing schizophrenia is having a first-degree relative [such as a brother] with the disease (risk is 6.5%)"

"Evidence supports a link between earlier onset of psychotic illness and cannabis use ... the more often cannabis is abused, the more likely a person is to develop a psychotic illness, with frequent use being correlated with twice the risk of psychosis and schizophrenia. Whether cannabis use is a contributory cause of schizophrenia, rather than a behavior that does not actually cause the disease, remains controversial."

You haven't done yourself any favors by using marijuana so much at a young age.  Changes to your intelligence, attention, and memory may be subtle enough that you don't notice them.  But they still might affect things over the course of your life.

Although the question of whether marijuana use causes schizophrenia, or makes it show up earlier, is still being debated, if we take the 6.5% likelihood of developing it because your brother already has it, and the suggested correlation that lots of marijuana use can double the chances, then you might have a 13% chance of developing the disease.  That may or may not seem like a big risk to you.  But add to that the mental effects that are apparently better documented, and you can imagine why your parents would be concerned.

Should you be as concerned?  Probably.  You have to take the possibilities seriously.  On the other hand, there are a whole lot of marijuana users out there, and they are not all walking zombies, with schizophrenia, paranoia and other psychoses, due to that use.  But they may not have started as young as you, or smoked it nearly as much.  You are still young enough that continued heavy use might cause more of the negative mental effects.  That might be reason enough to cut way back, if not quit completely.  If you do want to quit or cut back, and find you are unable to do so, you may need professional help to overcome your addictive habit.

You've still got most of your life ahead of you, with many wonderful things to experience.  Try to consider that when making decisions like this.  Good luck and best wishes!