Blood-Brain Barrier Integrity

The goal of this clinical trial is to better understand the relationship between blood-brain barrier (BBB) function and stress resilience and to investigate a potential causal role for BBB function in stress resilience in humans. The researchers will look for particpants that are young adults to answer the following questions to reach our goal: * Is better BBB integrity, measured with neuroimaging, associated with better stress resilience in the short term and in the long term? * Does the administration of Metformin improve BBB integrity? * Is improved BBB integrity a possible link between Metformin and better short- and long-term stress resilience? The participants will do the following during our study: * Participants will fill in online surveys on stressor exposure and mental health once every four weeks over 36 weeks * Participants will visit the study site 4 times: * The first time for a screening process which includes questionnaires, a medical exam and a blood sample. * The next 2 visits will include a blood sample, a medical exam, an MRI scan and a list of questionnaires * The last visit will include another blood sample, a medical exam and a list of questionnaires. * Between visit 2 and 3, the participant will take either Metformin or Placebo