Dentists typically recommend changing toothbrushes every three to four months, or sooner if the bristles become frayed or worn out. Over time, toothbrush bristles can deteriorate, making them less effective at removing plaque and bacteria from the teeth and gums. Additionally, bacteria can accumulate on the bristles over time, potentially leading to oral health issues if not replaced regularly. Therefore, it's important to monitor the condition of your toothbrush and replace it as needed to maintain optimal oral hygiene.