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Halitosis can be caused by various different reasons like bad oral hygiene
, dental caries
, gum disease
, stomach problem s , esophageal anomalies , liver dysfunction etc.,As you say you have constipation
, there could be an associated halitosis too.Please drink lots of water and take laxatives to avoid constipation.
You can reduce halitosis by maintaining oral hygeine.Cleaning tongue properly , eating a healthy breakfast and many dentists advise brushing teeth after your breakfast also keeps you fresh all the day.you can try a chewing gum.