It is essential to make the intention to fast the month of Ramadan at night before Fajr. It is not sufficient to start fasting during the day without the intention.

If a person starts fasting in one country and travels to another where Ramadan began later, they must continue fasting until the local community observes `Eid. This may result in fasting 31 days. However, if they move to a country where Ramadan ends earlier, they should break the fast with the locals and make up missed days later.

It is permissible to treat your toothache and apply paste and the like to the gums whilst fasting, but that is subject to the condition that you do not swallow it.