Unlike many prescription drugs, medical cannabis oil (concentrates) are completely natural and have few or no negative side effects. The amount of medical cannabis oil that you take, or your dose, will vary based on the type and severity of your condition, the level of experience you have with cannabis. The one important recommendation to follow: Start low and go slow. It is important to remember that cannabis oil taken orally has a slower onset time than when it is vaporized. Wait at least two hours after the first dose before taking a second of cannabis oil.