Do postnasal drips cause bad breath?

Yes: Especially if post nasal drip is caused by sinus infection.

Many causes: There are many causes of bad breath and post nasal drips are only one of them. Here is a website for more possible causes and information. http://smilesapartcosmeticdentist.com/fresh-breath-control.htm the most common cause and one of the easiest to treat is poor oral hygiene. Confirm the cause first and then see a dds or ENT for advice.