Hi.. Thanks for the query.. Well, if you continue to fast for long and stay starved then it can be a common cause for bad smell from mouth.. So in case if it has started since you started fasting then it can be most probably due to fasting and will resolve once you start eating and drinking normally.. Other common causes of bad smell from mouth can be poor oral hygiene, oral and dental infections,post nasal drip, sinusitis, acid reflux,respiratory infections, diabetes etc.. Once your fasting is over and if still you have bad breath then you cana should start gargling with a flavoured mouthwash twice a day.. You should chew flavored sugarfree chewing gum.. Do warm saline gargles twice a day.. Limit the intake of high aroma foods like onion and garlic.. Drink plenty of water and maintain a good oral hygiene.. Hope this helps.. Regards.