In my opinion, Medellin is one of the safest big cities in Colombia. Travelers like to go out in El Poblado or Laureles district. And many ex-pats live in Envigado. I wouldn't recommend hanging out near the center at night. Take a cab if you need to get there. Because many foreigners like to spend their money in expensive bars of Poblado, there are more problems with drugs, hookers, and Scopolamine. However, it's safe to walk around as long as the clubs are open. Also, don't hesitate to use the metro - Exception maybe for the last ones. Don't give Papaya, and you should be able to travel safely in Medellin.