They were right.

Most dandruff isn’t caused by dry scalp or poor hygiene but by Pityrosporum ovale, a yeast-like fungus that normally lives in harmony with the rest of the flora and fauna that inhabits our skin. Sometimes, though, the other microbes will die off, and P. ovale will take over in the same way that C. albicans will take over in cases of thrush mouth.

Listerine was at the time not just a mouthwash but an entire range of products, including a household antiseptic. Before antifungal medicines like miconazole and ketonazole were discovered, Listerine and other antiseptics such as Lysol and Dettol were the treatments of choice for any fungal or yeast infestation.

Nowadays if you have dandruff you can either use an antifungal cream or take an antifungal pill; dandruff shampoos, although popular, aren’t the treatment of choice. Often one pill is enough. (Scares the manly he-men, though, when the pharmacist hands over a package labelled “MONISTAT” in inch-high letters!)