In Jainism, the practice of fasting (upvaas, ayambil, etc.) is undertaken with great care to maintain purity and avoid any form of violence (himsa), including to the tiniest life forms. Whether one can brush teeth during a fast depends on the type and strictness of the fast, and the specific traditions followed.

Generally:

• During Upvaas (one-day fast without food but water is allowed): Brushing teeth with plain water is usually permitted, but using toothpaste or any consumable substance is discouraged, as it may break the fast due to ingestion of minute particles.
• During more rigorous fasts (e.g., Atthai, Chauvihar Upvaas): Even rinsing the mouth may be avoided, especially after sunset, to minimize the risk of swallowing any particles or harming microorganisms.
• Monks and nuns (Sadhu/Sadhvi): They use datun (natural neem sticks) and are extremely careful, often brushing before sunrise, and may refrain from brushing entirely during strict fasts.

Key Principles:

• Avoid ingesting anything (even accidentally).
• Avoid harming any life forms (even microscopic).
• Maintain spiritual discipline and purity.

Advice: If you are observing a fast, it is best to follow the guidance of your Jain guru, elders, or local tradition, as practices may vary among sects (Shwetambar, Digambar, etc.). When in doubt, use plain water, brush before sunrise, and maintain utmost care not to swallow anything.

In summary: Brushing teeth with plain water (not toothpaste) before sunrise is generally allowed during basic fasts, but stricter fasts may require even greater care or abstention. Always follow your tradition’s specific guidelines.