Salicylic acid, a promising agent against psoriasis?

Due to a malfunction of the immune system, the psoriasis causes skin inflammation and a faster than average renewal of skin cells. These dead cells accumulate on the surface of the epidermis, resulting in the appearance of raised skin plaques that can be red and itchy.

Seemingly, thesalicylic acid would help both to reduce the size and improve the appearance of plaques. Four studies focusing on this BHA as a monotherapy for the treatment of psoriasis plaques and two studies on salicylic acid used in combination with other treatments (topical corticosteroids, anthralin, calcineurin inhibitors, etc.) for more severe cases of psoriasis have been analysed.

• Forty patients suffering from scalp psoriasis were treated in a 21-day double-blind study with an alcohol-based lotion containing either betamethasone-17,21-dipropionate at 0.05%, or salicylic acid at 2%, or a combination of 0.05% of betamethasone-17,21-dipropionate and 2% salicylic acid, or a placebo. The results showed that the addition of salicylic acid enhanced the activity of the betamethasone-17,21-dipropionate.

• A randomised controlled trial from 1986 reported successful treatment for moderate or severe scalp psoriasis using a 6% salicylic acid gel alone in 30 patients (10 inpatients and 20 outpatients). The inpatients were treated for three weeks with daily applications of 6% salicylic acid, which were then removed 24 hours later with a standard shampoo, while the outpatients were treated for six weeks with salicylic acid applied once a day and removed 12 hours later. Nine inpatients (90%) and thirteen outpatients (65%) found that the salicylic acid helped to reduce the size of the affected areas on the scalp.

• A 21-day multicentre, randomised clinical study revealed that the topical application of a combination of 0.1% mometasone furoate and 5% salicylic acid for 7 days, followed by 14 days of 0.1% mometasone furoate (n = 184), was more effective than monotherapy with 0.1% mometasone furoate (n = 176).

• In 2009, a small open pilot study involving 20 individuals revealed that those treated for three to six weeks with 6% salicylic acid as a monotherapy once a day experienced a significant "clearing" of their scalp.

• A 6% salicylic acid was assessed in a small open pilot study involving 10 individuals suffering from scalp psoriasis. Six out of the ten participants (60%) saw their psoriatic plaques completely or almost completely disappear after four weeks of monotherapy.

• In a randomised, double-blind comparative clinical trial conducted with 25 patients suffering from moderate chronic psoriasis, researchers evaluated the therapeutic efficacy of a 6% salicylic acid cream compared to a 20% AHA/PHA emollient. Both formulas resulted in a significant reduction in scaling at the end of the two-week treatment period.

• A small clinical study from 2022 involving 20 subjects aged over 18 with mild to moderate psoriasis (<10% of the body surface area) showed that an over-the-counter lipophilic aqueous gel containing turmeric and salicylic acid (3%), followed by a shea butter exfoliating moisturiser with salicylic acid, improved the appearance of psoriasis plaques after 12 weeks of once-daily use.

According to published results, it is suggested that salicylic acid alone can to some extent alleviate symptoms (itching, irritation, erythema, etc.) and soften scales to facilitate their removal, although the studies have some limitations (no control group, small number of participants, etc.). However, while it cannot target the root cause of psoriasis, salicylic acid can act as an initial and complementary therapy to other psoriasis treatments.

By what mechanism?

It is suggested that the main benefit of salicylic acid in the treatment of psoriasis lies in its ability to promote the removal of corneocytes through two possible pathways:

1. it reduces the adhesion and cohesion between dead cells by dissolving the intercellular cement (keratolytic effect);

2. it lowers the pH of the stratum corneum, which increases its hydration.

However, salicylic acid-based products only target the physical plaques and not the immune processes that lead to skin inflammation and hyperproliferation of skin cells. A systemic medication would be needed in addition to control the disease from an autoimmune perspective. Nevertheless, by detaching the links between the cells of the stratum corneum, it increases the penetration rate of other topical therapies for psoriasis and enhances their availability. Salicylic acid also has the ability to reduce inflammation, which helps to alleviate the itching and redness associated with psoriasis.

A 2015 review warns that salicylic acid is not an appropriate treatment for young children suffering from psoriasis due to a higher risk of systemic absorption and the negative reactions it presents. However, it appears to be safe in pregnant women with localised psoriasis.