Taboo and Exemption during fasting

Related to Head & Mouth:
1. Eating & Drinking
2. Intentionally letting the thick dust or smoke enter the throat (Smoking)
3. Vomiting intentionally (even if it is for a medical purpose)
4. Submerging the head completely into the water

Related to Private Parts:
5. Sexual intercourse
6. Intentionally remaining in the state of period till the time of fajr
7. Doing anything that would cause discharge of semen
8. Using liquid enema

Moral/Ethical:
9. lying of the highest type — falsely attributing a statement to Allãh, the Prophets, and the Imams — makes the fast bãtil.

Exempted from Fasting
1. An elderly person who cannot fast at all or it causes great difficulty for him/her.
2. A person whose medical condition does not allow him/her to stay thirsty
3. A pregnant or a nursing woman if fasting would harm her or the child.

Source: Things that are forbidden in fasting