Which active ingredient combinations should you avoid?

In another study, a serum combining glycolic acid and salicylic acid was tested in 66 patients with inflammatory or cystic acne. After two weeks of daily application, more than 90% of patients experienced a reduction in their acne, an improvement in their skin texture and a decrease in their hyperseborrhea. The authors did not specify the concentrations used or detail observed adverse effects, which limits interpretation of the results.

If combining exfoliating agents may offer clinical benefits, it remains inadvisable for daily use in sensitive skin. To limit irritation risk, alternate applications every other evening rather than apply both at once.

Combining exfoliants such as AHAs or BHAs with pure vitamin C can irritate sensitive skin. For reference, the vitamin C pure form (INCI: Ascorbic Acid) is acidic with a pH below 3.5. This low pH is required to ensure stability and antioxidant efficacy. It can weaken the skin barrier and cause redness and stinging. For this reason, some formulations incorporate derivatives of vitamin C, less irritating. When used with exfoliating actives, pure vitamin C amplifies their keratolytic action and increases skin sensitivity.

However, as with the simultaneous use of exfoliants, combining vitamin C with exfoliants may aid in reducing wrinkles and pigmentation spots. In a study conducted in Singapore on 15 patients with signs of photoaging, chemical peels using 70% glycolic acid combined with vitamin C at an undisclosed concentration were performed three times at four-week intervals. After twelve weeks, skin texture and radiance showed visible improvement. However, common side effects such as redness and swelling, were reported by several patients and persisted in the weeks following the final treatment. These side effects were assessed on a scale from 1 to 4, with (1) no side effects, (2) mild side effects, (3) moderate side effects, and (4) severe side effects.

Pure vitamin C can be combined with exfoliants to help prevent photoaging or reduce blemishes. However, this combination can irritate sensitive skin.

The retinol and the benzoyl peroxide are two actives used in acne management. Each has a distinct mechanism of action. Retinol promotes cell turnover and normalizes keratinization, preventing pore blockage. Benzoyl peroxide targets the bacterium Cutibacterium acnes, the species involved in inflammatory lesion formation, while exhibiting a mild keratolytic effect. Guidelines advise against combining retinol and benzoyl peroxide due to a potential interaction: benzoyl peroxide, as an oxidizing agent, may oxidize retinol, reducing its efficacy and increasing the risk of irritation.

Despite these warnings, some recent data suggest that, in well-balanced formulations, the retinol–benzoyl peroxide duo can improve mild acne. For instance, a clinical study evaluated the efficacy of a cream combining 4% benzoyl peroxide, 0.05% retinol, 0.5% retinyl palmitate, 1% mandelic acid, and glycyrrhetinic acid. Over twelve weeks, 20 patients with mild acne applied this cream once daily to their face. The results showed a good overall tolerability, with limited and transient irritation. Moreover, an average reduction of 65% in comedones, 62% in papular lesions, and 80% in pustular lesions was observed.

It appears that, at appropriate doses, the combination of retinol and benzoyl peroxide provides an effective option for managing mild acne. However, as a precaution, introduce these actives in stages. For example, alternate their application every other night at first.

Dihydroxyacetone (DHA) is the active ingredient most often used in sunless tanners. Its mechanism is based on the Maillard reaction. DHA reacts with free amino groups on stratum corneum proteins, including arginine residues. This reaction forms brown pigments called melanoidins. They give the skin a tan appearance. Formulations may include peptides. These short chains of amino acids stimulate collagen synthesis and reduce wrinkles. However, peptides also contain free amino groups.

We can assume that the DHA reacts with peptides in a way similar to its reaction with stratum corneum proteins. This may alter peptide structure and reduce their activity or render them inactive. At the same time, DHA itself would react less with skin proteins, decreasing its self-tanning effect. These are hypotheses based on the potential chemical reactivity of these molecules: to date, no study has confirmed this interaction, but it remains a point of caution in formulation. To avoid compromising the efficacy of your self-tanner and peptide treatment, we recommend applying them at different times.