Anti-dandruff shampoo: becoming less and less effective?

How do anti-dandruff shampoos work?

Dandruff is due to an imbalance in the scalp's cellular renewal process. Normally, keratinocytes take about a month to differentiate before shedding as invisible corneocytes. In individuals prone to dandruff, this cycle is accelerated to 7–10 days, leading to an accumulation of poorly cohesive cells visible to the naked eye. This phenomenon is associated with the proliferation of a lipophilic fungus of the genus Malassezia, which hydrolyses sebum triglycerides into irritating fatty acids, especially oleic acid. These acids impair the skin barrier, increase transepidermal water loss and elicit mild inflammation, thereby intensifying itching and promoting flake formation.

Anti-dandruff shampoos primarily address the microbial and inflammatory imbalance. Antifungal actives are the cornerstone and directly target Malassezia. Ketoconazole, an imidazole derivative, is one of the most extensively studied: it inhibits ergosterol synthesis, a component of yeast cell membranes, leading to their weakening. Selenium sulphide, for its part, combines an antifungal action with a cytostatic effect, slowing the proliferation of scalp keratinocytes. Climbazole and zinc pyrithione are also used to limit fungal growth and soothe local inflammation. To these classics are added two other molecules also used in cosmetics: the ciclopirox olamine, which acts by chelating the metal ions necessary for the enzymes of Malassezia and also has an anti-inflammatory effect, and the piroctone olamine, which inhibits the yeasts’ energy metabolism by blocking the utilisation of iron.

Besides their antifungal action, some anti-dandruff shampoos incorporate keratolytic agents. The salicylic acid, for example, breaks down corneocyte junctions and facilitates the removal of scales. Coal tar, now used less often for reasons of tolerability and odour, acts by slowing epidermal proliferation. Finally, certain agents such as lactic acid or urea contribute to restoring hydration of the stratum corneum, enhancing scalp comfort.

The efficacy of anti-dandruff shampoos relies on a combination of actions: inhibition of the yeast Malassezia, reduction of inflammation, normalisation of the keratinisation cycle and support for the skin barrier function.

Dandruff

R12 — Anti-Dandruff Treatment Shampoo
1% Piroctone Olamine + Jujube Extract — £21.90

Do anti-dandruff shampoos lose their efficacy with repeated use?

Contrary to a common misconception, anti-dandruff shampoos retain their efficacy over time when used regularly and in accordance with recommended guidelines.

Available studies show that antifungal agents such as selenium sulfide, zinc pyrithione, ketoconazole and ciclopirox olamine retain their effectiveness over successive applications, sustainably reducing flaking, itching and the visible signs of dandruff. In reality, the opposite often occurs: stopping shampoo use quickly leads to a recurrence of symptoms, as the scalp returns to its original imbalance. This confirms that the regular use is essential to maintain the benefits.

Several clinical trials aptly demonstrate this phenomenon. In a 1998 study by GOODE and his team involving 163 individuals with dandruff or seborrhoeic dermatitis, applying a shampoo containing 1.5% ciclopirox olamine or 2% ketoconazole three times weekly for four weeks led to a 78% and 90% reduction in the dandruff severity score, respectively. The efficacy was maintained throughout the protocol, but discontinuing treatment for two weeks led to the recurrence of dandruff. Similarly, another study by the same researchers in 2002 on 154 participants, including 70 with seborrhoeic dermatitis, compared a shampoo combining 1.5% ciclopirox olamine and 3% salicylic acid with one containing 2% ketoconazole. After four weeks, dandruff had decreased by 22% with the former and 26% with the latter, while the area affected by seborrhoeic dermatitis was reduced by 62.5% and 80.5%, respectively. Again, efficacy was confirmed throughout the study, but discontinuing the shampoo for two weeks resulted in a progressive return of symptoms.

Évolution de la sévérité des pellicules pendant et après application de shampooings anti-pelliculaires avec A (GOODE & al. 1998) et B (GOODE & al. 2002). — Evolution of dandruff severity during and after application of anti-dandruff shampoos containing A (Goode et al. 1998) and B (Goode et al. 2002).
Sources: GOODE K. & al. Comparative clinical efficacy of shampoos containing ciclopirox olamine (1.5%) or ketoconazole (2%; Nizoral) for the control of dandruff/seborrhoeic dermatitis. Journal of Dermatological Treatment (1998). / GOODE K. & al. A randomised, single-blind, single-centre clinical trial to evaluate comparative clinical efficacy of shampoos containing ciclopirox olamine (1.5%) and salicylic acid (3%), or ketoconazole (2%, Nizoral ) for the treatment of dandruff/seborrhoeic dermatitis. Journal of Dermatological Treatment (2002).

The perception that some users have of their shampoo becoming less effective over time has no scientific basis. It can be explained by several factors: inconsistent use, physiological changes in the scalp (sebum levels, microbiota, inflammatory state), or the chronic and recurrent nature of dandruff itself. These natural fluctuations can give the impression of reduced effectiveness, whereas the active ingredients maintain their antifungal action. Thus, no data suggests that the scalp develops resistance to the active ingredients, unlike what can be observed with antibiotics in bacteria. The key therefore remains consistency in the use of anti-dandruff shampoos.