How soon is birth control effective at stopping pregnancy?

On my information pamphlet it says if you start on the first day of your period you are protected immediately and no alternative forms of birth control are needed. However, upon researching further just to be sure, I saw many people saying you had to wait at least a month. FYI, the pills are Tri Sprintec. Why the contradictions, and can I trust the information in the pamphlet.