Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Our entire understanding of the cause of dandruff shifted with this landmark article published in 1984. Instead of relying on secondary sources, reviews, editorials, and opinion pieces, he looked at the primary literature, the original studies, and “was amazed to find out how overwhelming was the evidence of the [true cause], and how it had been ignored because it was so well buried under the mountain of error” since some expert in the 1800s put forth some bogus theory.

We now know that dandruff is triggered by a fungus that lives and feeds on the human scalp—the two major implications being, first, how “alarming” it is that a bogus theory can remain in the medical literature unchallenged for a century despite evidence to the contrary; and, second, hey, if it’s a fungus, what about trying tea tree oil, which contains components that have “antifungal activity” against a range of fungi.

That was based on studies like this, though, where tea tree oil in a petri dish can fight off pathogenic skin fungi. But, you don’t know if it works for dandruff, until you put it to the test. A hundred and twenty-six men and women randomized to daily use of a “5% tea tree oil shampoo or placebo” for a month. The placebo worked a little bit, decreasing dandruff severity by about 10%, but the tea tree oil shampoo worked better—about a 40% drop. Looks like more than 40% from the graph, but that’s because they misleadingly started the Y-axis at -60. This is a classic deception featured in chapter 5 of the 1954 classic How to Lie with Statistics. The graph should really look like this, which makes the effect less impressive, but it was still statistically significant.

“[O]nly one patient [in the tea tree oil group] actually achieved a complete response,” though one in the placebo group did as well. Thus, it appears that the “tea tree oil shampoo would require ongoing application for control of dandruff.”

Speaking of fungus, what about tea tree oil in the treatment of athlete’s foot? That may actually be our most common fungal skin infection, affecting up to one in ten. So, about a hundred patients randomized into one of three groups, a 10% tea tree oil cream, tinactin (an antifungal drug), or a placebo cream. A month later, the fungus was wiped out in 85% of the drug group, but only about a quarter of the placebo and tea tree oil groups. This is somewhat surprising, since tea tree oil can kill off the fungus in a petri dish—but, apparently, not on toes.

That reminds me of some of the oral health data on tea tree oil. It can wipe out some oral pathogens in a petri dish, but have people swish a tea tree oil solution around in their mouth, and here’s the dental plaque buildup after 4 days of no brushing swishing with a placebo. Here’s swishing with a medicated chlorhexidine mouthwash, which keeps the plaque a bit at bay, but the tea tree oil mouth rinse? No effect.

So, if tea tree oil doesn’t influence the amount of plaque, presumably it wouldn’t help with gingivitis, the gum inflammation that’s caused by plaque buildup. But, no; here’s the twist. True, no reduction in plaque with a 2.5% tea tree oil gel—yet “significant reduction” in gingivitis scores. Since decreased gum inflammation occurred without a decrease in plaque, it appeared to just be helping more from an anti-inflammatory rather than antimicrobial mechanism.

Might the same thing be happening here? Yeah, from a mycological cure standpoint—a fungus cure standpoint—tea tree oil didn’t really do any better than placebo. But, though the drug wiped out the fungus in 85% of cases, in some of those cases, the patients actually didn’t notice an improvement in symptoms, or they actually felt worse after the drug—probably a reflection of tinactin’s “irritant side effect[s].” If instead of mycological cure, you looked at symptom improvement, tea tree oil works as well as the dru g . So, “[t]his may be the basis for the popular use of tea tree oil in the treatment of [athlete’s foot].” But, people should realize that it’s just symptomatic relief, and they’re not necessarily eliminating the underlying cause. Of course, maybe they didn’t use a strong enough concentration.

And, indeed, if you go with not a 10% cream, but up to 25 or 50%, you can get “mycological cure rate[s]” above that of placebo, but still not as good as the drug. And, at those high concentrations, some of the patients applying tea tree oil “developed moderate to severe dermatitis”—they broke out in a rash. But, hey, if you have a patient that doesn’t want to use the medicated creams, then a 25% tea tree oil application has a decent chance of knocking it out without being too risky. But, the standard over-the-counter antifungal creams may work better.

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