Fluoride varnish
Fluoride varnish is a highly concentrated form of fluoride which is applied to the tooth's surface, by a dentist, dental hygienist or other health care professional, as a type of topical fluoride therapy. It is not a permanent varnish but due to its adherent nature it is able to stay in contact with the tooth surface for several hours. It may be applied to the enamel, dentine or cementum of the tooth and can be used to help prevent decay, remineralise the tooth surface and to treat dentine hypersensitivity. There are more than 30 fluoride-containing varnish products on the market today, and they have varying compositions and delivery systems. These compositional differences lead to widely variable pharmacokinetics, the effects of which remain largely untested clinically.

Fluoride varnishes are relatively new in the United States, but they have been widely used in western Europe, Canada, South Africa and the Scandinavian countries since the 1980s as a dental caries prevention therapy. They are recognised by the Food and Drug Administration for use as desensitising agents, but, currently, not as an anti-decay agent. Both Canadian and European studies have reported that fluoride varnish is as effective in preventing tooth decay as professionally applied fluoride gel; however, it is not in widespread use for this purpose.

Fluoride varnish is composed of a high concentration of fluoride as a salt or silane preparation in a fast drying, alcohol and resin based solution. The concentration, form of fluoride, and dispensing method may vary depending on the manufacturer. While most fluoride varnishes contain 5% sodium fluoride at least one brand of fluoride varnish contains 1% difluorsilane in a polyurethane base and one brand contains 2.5% sodium fluoride that has been milled to perform similar to 5% sodium fluoride products in a shellac base.

There is low quality evidence that resin-based pit and fissure sealants were more effective in preventing occlusal caries than fluoride varnish.   This same review provided low quality conclusions regarding superior performance when tooth surfaces were sealed and varnished with fluoride, as opposed to when varnished only with fluoride.

Clinical recommendations
A panel of experts convened by the American Dental Association (ADA) Council on Scientific Affairs presents evidence-based clinical recommendations regarding professionally applied, prescription-strength and home-use topical fluoride agents for caries prevention. The panel recommends the use of 2.26 percent fluoride varnish for people at risk of developing dental caries. As part of the evidence-based approach to care, these clinical recommendations should be integrated with a practitioner's professional judgment and the patient's needs and preferences.

United Kingdom 
Fluoride varnish is widely used in the United Kingdom, following guidelines from multiple sources backing its efficacy. Public Health England, a UK government organisation sponsored by the Department of Health, released guidance in 2014 recommending fluoride varnish application at least twice yearly for children and young adults. Similarly, the Scottish Intercollegiate Guidelines Network and the Scottish Dental Clinical Effectiveness Programme have both released independent guidance recommending at least twice yearly fluoride varnish application, citing a strong clinical evidence base. SIGN recommends fluoride varnish at a concentration of 2.2%, while SDCEP recommends 5%.

Types of varnish
Different varnish products release varying amounts of calcium, inorganic phosphate, and fluoride ions. MI varnish releases the most amounts of calcium ions and fluoride. Enamel Pro varnish releases the most amount of inorganic phosphate ions. Each type of varnish is designed to be used in specific situations.

Cost effectiveness
Fluoride varnish treatment has a better outcome at preventing cavities at a lower cost compared to other fluoride treatments such as the fluoride mouth rinsing. 
For fluoride varnish treatment, the benefit to cost ratio 1.8:1, whereas fluoride mouth rinsing is 0.9:1. With fluoride varnish treatments, one can save by preventing future restorations. Fluoride varnish also requires fewer treatments for measurable effectiveness, therefore in the long run it is cost effective when compared to other treatments.

Advantages and disadvantages

Advantages
Fluoride varnishes are available in different flavours which can be advantageous when treating younger patients 
They dry rapidly and will set even in the presence of saliva
Because they do not require the use of fluoride trays they are suitable for use in patients with a strong gag reflex (See image to the right)
Due to the small amounts used and the rapid setting time there is only a small or negligible amount of fluoride ingested 
It has a sticky consistency which helps it to adhere to the tooth’s surface thereby allowing the fluoride to stay in contact with the tooth for several hours
Based on published findings, professionally applied fluoride varnish does not appear to be a risk factor for dental fluorosis, even in children under the age of 6. This is due to the reduction in the amount of fluoride which may potentially be swallowed during the fluoride treatment because of the small quantities used and the adherence of the varnish to the teeth.
Fluoride varnish treatments are shown to reduce the number of the cariogenic bacteria S. Mutans by over ten-fold.
Fluoride varnish was a higher concentration than the foam and gel.  There was not a significant difference in the amount of remineralization between gels, foams, and varnish.  A study with a larger sample size and a longer time frame could show differing results.
They can be applied easily and quickly

Disadvantages
Due to the color and adherence of most fluoride varnishes, they may cause a temporary change in the surface color of teeth as well as some filling materials. As the varnish is worn away by eating and brushing, the yellowish colour fades.
Varnish costs more than gel and requires a prescription unlike the gel that is over the counter.
They do not have the bitter taste of some fluoride gels, but in some patients the taste of the varnish can cause nausea especially when consuming food within the 24 hours post treatment.

Indications and contraindications

Indications for use 

 Use as a topical fluoride agent on moderate and high-risk patients, especially children aged 5 and younger 
 Desensitizing agent for exposed root surfaces
 Fluoridated cavity varnish
 When a higher concentration of fluoride is needed for high caries risk patients
 In the elderly to prevent increasingly prevalent root dentin lesions, which may require higher concentration of fluoride
 On advanced enamel carious lesions, which may also require higher fluoride concentration for remineralization  
 Fluoride treatment for institutionalized patients or in other situations where setting, equipment and patient management might preclude the use of other fluoride delivery methods
 Caries prevention on exposed root surfaces
 Remineralization of lesions in root dentin 
 Fluoride application around orthodontic bands and brackets (See image to right)
 Fluoride treatment on patients when there is a concern that a fluoride rinse, gel or foam might be swallowed

Contraindications for use 
Areas with open cavities 
Patients that are at low-risk or are decay-free and live in an area where the water is fluoridated
Treatment of areas where discoloration after treatment may be an aesthetic concern