There's some intriguing, yet preliminary, evidence to back up those of thousands of years of observational antibacterial and anti-inflammatory benefits.
Research into the practice of oil pulling shows that indeed the daily swish may be as effective -- or almost as effective -- as chlorhexidine mouthwash against bad breath, plaque, receding gums and gum disease, as well as against the cavity-causing bacteria Streptococcus mutans (S. mutans) [sources: Asokan, Asokan, Asokan]. Let's put that into perspective. Chlorhexidine mouthwash is a prescription-grade antimicrobial and antiseptic (chemical) rinse used to treat gum disease. It's more powerful than an over-the-counter version intended to freshen your breath. And swishing with a tablespoon of sesame oil is observed to have similar benefits -- but unlike chlorhexidine mouthwash, which can be associated with staining, sesame oil used in oil pulling may whiten teeth.
Sesame oil (unrefined, and not toasted) is most traditionally used in oil pulling because this polyunsaturated fat has some notable health-beneficial components -- including magnesium (for lowering blood pressure and glucose levels, as well for good respiratory health), zinc and copper for bone health, and antioxidant and anti-inflammatory properties.
The antimicrobial properties of coconut oil (which is solid at room temperature, be aware) are also showing promise in limited studies. One small study conducted at Loma Linda University found that the mouths of participants who pulled with sesame oil had five times fewer bad bacteria than those who pulled only with water; and pulling with coconut oil showed benefits, too. The mouths of these participants had two times fewer bacteria [source: Almendrala]. Additionally, coconut oil is full of lauric acid, a known antimicrobial agent.
While there's no evidence that the plaque in your mouth will dissolve in fat (as some proponents of oil pulling believe), some theories suggest that bacteria does attach to the oil-saliva mixture formed in your mouth. It may be attracted to the oil, but saliva does more than just keep your mouth moist. It's antibacterial, which means it fights germs in your mouth, and it's rich in salivary proteins that fight cavities, as well as minerals to strengthen tooth enamel [source: Dowd]. Adding oil seems to reduce the number of bacteria in the mouth, but it's important to note that studies to show the science behind the anecdotal evidence are limited. Ongoing research to prove any benefits oil pulling may have will need to be done with a wider audience. In the meantime, though, if you can stomach swishing with sesame or coconut oil, no harm done -- as long as you keep up with your regular brushing and flossing habits (and don't swallow that oil).