The answer to this question really depends on how much marijuana someone uses, how often, and how dependent someone is on it. The best thing to do is to discuss this question with your doctor
There are a few medical effects of long term marijuana use that have been shown to be true. The effects of smoking marijuana on the lungs are similar to the effects of smoking cigarettes- the main difference being that people tend to smoke a lot less marijuana than smoke, so that hazardous effects of smoking and the bad stuff in the smoke (like tar) are less toxic. In general, people who smoke marijuana just every once in a while do not see long term bad effects on their lung; That being said marijuana has been shown in some studies to be associated with an increased long term risk of lung cancer.
The main risk of long term use of Marijuana is what it does to someone's behavior patterns, such as professional and school success or the ability to sustain interpersonal relationships. Occasionally people can become dependent on the drug to feel good, in which case they begin to abandon other things they used to have interest in. Also there is some evidence that long term use of marijuana can effect the users long term memory or what is called in medical terms "cognitive effects." In addition, in people with a propensity towards or predispostion towards psychosis or other mental health issues, marijuana can exacerbate the mental health issues.
Of course, if you have any questions or concerns at all, it is most prudent to see your doctor about this. Please make an appointment to speak to your doctor regarding this issue.