isolated one

As long as you are sure there's no unknown physical condition or poor diet causing your loss of energy, a multi would be a good start. But don't expect instant results, as this has to be taken on a regular basis for benefit. If you're interested, there are some herbs which can provide a more instant boost, along with long term benefits as well. I have found Cayenne and Korean Ginseng to be valuable in this area. However, I wouldn't take either one until you are able to become familiar with how they work or if you happen to be taking medication.