OBJECTIVE: Limited evidence has suggested that a worn toothbrush may be significantly less effective than a new brush with respect to plaque removal. Two independent studies, one with a manual toothbrush and one with a powered toothbrush, were undertaken to compared the ability of these toothbrushes as new and after three months' use to remove plaque. [...]
CONCLUSION: The results from these two studies provide further data in support of the hypothesis that a worn toothbrush is less efficient with respect to plaque removal than a new brush. Patients should therefore be encouraged to replace their toothbrush regularly before bristle wear becomes excessive.
A toothbrush should be replaced every three or four months, or sooner if the bristles become frayed. A worn toothbrush does not do a good job of cleaning your teeth.