It depends. If you are a younger person (25 years or younger roughly) the problem you are trying to prevent is decay developing between teeth. This normally occurs at the contact point between the teeth. If there is enough plaque (and it’s acid) sitting in there it will begin to dissolve the enamel surface and eventually form a cavity. If you are using your toothbrush regularly, properly and thoroughly it will help minimize the plaque accumulation around the teeth at the gum line. However, it won’t get into the tight “in-between” spots. Therefore you need to floss.
On the other hand if you are older and/or have some issues with gum recession or periodontal (gum) problems, you likely are no longer as prone to the cavities of youth but can accumulate more plaque between the larger spaces between the teeth. This in turn exacerbates the gum problem and can also lead to root surface decay. Using only floss in this situation doesn’t really do much compared to using interdental brushes. This was shown in a 2007 study (Dental Abstract May/June 2007 issue) of 77 patients with periodontal disease. After being cleaned up, these people were either instructed to use floss or use interdental brushes. Follow up of both groups was done at 6 and 12 weeks. Both groups showed plaque reductions but the group using the interdental brushes had significantly and consistently better results.