Pilot Study Repetitive Transcranial Magnetic Stimulation (rTMS) in Cannabis Craving

Background: * Cannabis, also known as marijuana, is a commonly abused drug. There is no approved medication to treat cannabis addiction. The desire to use cannabis often increases when seeing others use it or seeing pictures of it. Researchers are interested in determining which parts of the brain are active when looking at pictures associated with cannabis. * Repetitive transcranial magnetic stimulation (rTMS) uses magnetic pulses to stimulate the brain. These pulses can change activity in parts of the brain. Researchers are interested in determining whether rTMS can decrease activity in the parts of the brain that respond to cannabis, and thereby lessen cravings for cannabis. Objectives: \- To determine whether transcranial magnetic stimulation can lower craving for cannabis when people who use cannabis are shown images that increase craving. Eligibility: \- Individuals 18 years of age and older who are physically healthy and currently use cannabis (at least 3 times weekly for the past 2 years). Design: * The study will involve eight visits over 3 weeks, with each visit lasting 1 to 2 hours. * Participants will have an initial assessment about cannabis use and provide blood and urine samples before beginning the study. * Participants will have three magnetic resonance imaging (MRI) brain scans. The first MRI scan will look at the structure of the brain at rest. During the other two scans, participants will look at images related to and not related to cannabis use. * The rTMS sessions (with either actual TMS or inactive \[sham\] TMS) will be held daily for 5 consecutive days. During these testing visits, researchers will ask questions related to drug use and craving, and collect urine and breath samples for further study. * Participants will have two follow-up visits, 1 week and 2 weeks after the rTMS sessions, to evaluate memory and mood, and one final MRI brain scan at the end of the study.