Is it better to floss before or after brushing the teeth?

Question

Floss: Dental floss is a cord of thin filaments used to remove food and dental plaque from between teeth. The floss is gently inserted between the teeth and wiped along the teeth sides, especially close to the gums or underneath them. In dentistry, floss is classed as an interdental (between teeth) cleaning aid.

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On various youtube video I hear opposite opinions regarding the best order of flossing and brushing. And if we're at it, then what about mouthwash?

I thought that the best practice is to go from the coarse to the fine, i.e. first brush, then floss, then mouthwash.

• Dr. Mark Burhenne ("Ask the Dentist") - How to Floss the Right Way - argues brush then floss.

• Dr. Scott Frey - FreySmiles argues Floss then brush then mouthwash

• Dr. Zak - everydayHealth argues it doesn't matter.

• Sharma et. al., 2004 recommends brush then floss then mouthwash.
• Zimmer et. al., 2006 recommends mouthwash instead of floss

Answer 1

Bottom line: It doesn't matter if you floss before or after brushing your teeth.

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This question has been debated endlessly within the dental community. On this site it is really hard to conclude which one is better due to many different papers like you cited and many different points of views from dentists and professionals.

I will cite one answer only from the American Dental Assosiation, the nation's largest dental association and the leading source of oral health related information for dentists:

The American Dental Association has concluded is that it’s fine to do it in both ways. What sequence you use doesn't really matter as long as you do the job, they reported:

Either way is acceptable as long as you do a thorough job. However, if you use dental floss before you brush, the fluoride from the toothpaste has a better chance of reaching between teeth. Some people brush their teeth and unfortunately skip flossing because they think their mouth feels clean or they may be short on time or tired and flossing is postponed. That’s not a good idea.

It also seems that ADA has come up with this conclusion because as it seems there is no conlcusive evidence on which one is better, dentabout.com reported:

There is no hard evidence as to whether it’s better to floss before or after you brush, therefore it doesn’t really matter.

Answer 2

The ADA's stance is overly diplomatic because it is a political entity and not a scientific resource so I wouldn't expect them to provide productive clinical guidance one way or another. Creating well controlled longitudinal studies with a sufficient sample size to evaluate the clinical benefit of a particular oral hygiene regimen is unreasonable and never going to happen so we are left to rely on the literature detailing the pertinent biochemistry to base our clinical decisions.

The bottom line is that fluoride needs to penetrate into the plaque fluid effectively to do its job (much of this is impeded by plaque thickness). Since your toothbrush bristles don't get all the way between teeth, the best way to maximize its bioavailability is to floss and then brush. Ideally this is followed by a fluoridated mouthwash instead of rinsing with water to maximize the benefit of fluoride. Toothpaste is has good substantivity and a higher fluoride concentration that OTC fluoridated mouthwashes so it is the key fluoride delivery vehicle in the process.

The first study cited in your question is an industry sponsored paper meant to help sell Listerine, the second doesn't quite conclude that mouthwashes are a substitute for floss merely that they are a key component of a well rounded hygiene regimen.

Combing through the evidence critically overwhelmingly supports the fact that floss, brush, mouthwash produces the most effective biochemistry for fluoride to do its job.

Edit: I guess it serves me right for not citing my comments in a skeptics forum. Here are a few relevant citations.

1. This study and others show the synergistic effect of a fluoridated mouthwash after brushing: Duckworth, R. M., Horay, C., Huntington, E., & Mehta, V. (2009). Effects of flossing and rinsing with a fluoridated mouthwash after brushing with a fluoridated toothpaste on salivary fluoride clearance. Caries Research, 43(5), 387–390.
2. Limitations of penetrance of fluoride into plaque biofilms: Watson, P. S., et al. "Penetration of fluoride into natural plaque biofilms." Journal of dental research 84.5 (2005): 451-455.