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I wonder how frequently the use of topical imiquimod cream results in permanent scarring and/or skin discoloration (e.g., hyper- or hypopigmentation). What I have found so far doesn't give any source supporting their claims and is sometimes contradictory:

https://www.rxlist.com/aldara-side-effects-drug-center.htm:

Changes in skin color of the treated area may occur and may not go away.

https://www.mayoclinic.org/drugs-supplements/imiquimod-topical-route/side-effects/drg-20067474:

Less common [side effect]: white, yellow or waxy scar-like area

https://www.aad.org/public/diseases/skin-cancer/imiquimod-skin-cancer-treatment:

  • Little scarring
  • No skin discoloration

https://www.ouh.nhs.uk/patient-guide/leaflets/files/14441Paldara.pdf (mirror) {1}:

Allergic reactions to Aldara are rare. If you have a severe reaction, there is a small risk of scarring or hair loss. Rarely, Aldara can cause a flare up of a pre-existing autoimmune disease.

(note: Aldara is a US Brand Name for 5% imiquimod cream and Zyclara is for 2.5% and 3.75%)

https://somepomed.org/articulos/contents/mobipreview.htm?34/35/35387{1}:

Topical 5-fluorouracil and imiquimod: Noninvasive. Rarely causes scarring. Avoids operative risks. Able to use in patients who are otherwise not candidates for surgery.

How frequently does the use of topical imiquimod cream (e.g., to treat a superficial basal cell carcinoma (BCC)) result in permanent scarring and/or skin discoloration? I'm looking for a percentage from some study instead of a vague, unsupported "rare", "sometimes", etc. that the quotes above give

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The studies I found very significantly contradict each other.

The 2020 systematic review {6} states that the risk of after the use of topical imiquimod 5% cream to treat a nodular basal cell carcinoma (BCC) is ~2%:

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The 2009 systematic review {5} summarized the likelihood of scarring and/or skin discoloration (e.g., hyper- or hypopigmentation) after the use of topical imiquimod 5% cream to treat a basal cell carcinoma (BCC):

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The 2018 study {4} states 0% of permanent hypopigmentation/depigmentation:

Hypopigmentation depigmentation was present in 71% (17/24) of the lesions immediately after treatment and 6 months later. Hypopigmentation/depigmentation eventually healed, with normal skin pigmentation, in all patients.

Whereas 67% of treated cases of BCC treated by imiquimod 5% cream had hypopigmentation in this 2007 study {1}.

The cosmetic results were excellent in 33% of cases (n = 24), but a permanent hypopigmented macule was observed in 67% of cases (n = 12) (Figure 2).

Important note: most patients in this study have a skin type 3 on the Fitzpatrick scale, which may strongly the impact the percentage of hypopigmentation cases, see below for more information.

Example of hypopigmentation following a treatment of BCC treated by imiquimod 5% cream (see last picture C):

enter image description here

Limitations:

  1. Small sample size.

  2. Two authors of the study have disclosed a conflict of interest (paid by 3M Pharmaceuticals, which manufactures Imiquimod).

  3. The study was published in 2007. Hopefully larger, more objective studies have been published since then.

  4. Patients had different skin types, as assessed with the Fitzpatrick scale. I was told by a dermatologist the darker the skin, the more likely it is to get hypopigmentation from a imiquimod 5% cream. To be confirmed with a study. The following table shows the skin type distributions in the {1} study:

    enter image description here

    enter image description here

Regarding the location of the hypopigmentation, all studies I have found state the hypopigmentation, if any, is mostly limited to the site of application. Quote from {2}:

Imiquimod-induced pigment loss has mainly been limited to the site of application.

which collaborates {3}:

Patient with multiple hypopigmented macules coalescing into large hypopigmented patches at the site of topical imiquimod therapy

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