Use O2 as often as you need. Listen to your bodily requirements and responses, and take it when you sense that you need to. Most people take O2 just once a day, first thing in the morning. When feeling run down or stressed take it twice or three times a day. If you have to do some strenuous work or sport, if you have to stay up late at night to work or any other activity that requires concentration, you can take an extra dose of O2 beforehand to give you additional stamina.