Hello, Thank you for your question regarding the use of a 0.5 mm dermaroller on stretch marks on your breasts. It is generally safe to use a 0.5 mm dermaroller on stretch marks up to twice a week. However, the frequency can vary based on your skin's sensitivity and how it responds to the treatment. Some individuals may tolerate more frequent use, while others may need to limit it to once a week or even less often to avoid irritation. It's important to monitor your skin's reaction and give it adequate time to heal between sessions. If you experience excessive redness, irritation, or discomfort, it may be beneficial to reduce the frequency. Additionally, always ensure that you are following proper sterilization procedures for the dermaroller to prevent infections. For personalized advice, I recommend consulting with a dermatologist who can assess your skin's condition and provide tailored recommendations. Best regards,