Why has TPO, which is so prevalent in semi-permanent nail varnishes, now been banned?

Diphenyl trimethylbenzoyl phosphine oxide (TPO) is a chemical compound belonging to the family of photoinitiators. Its primary function is to initiate the polymerisation of gels applied to the nails when exposed to a UV or LED lamp. In other words, it is thanks to TPO that the semi-permanent varnishes harden rapidly and acquire their much-appreciated durability. This property has greatly contributed to the success of these products in nail salons, as it allows for long-lasting wear of the varnishes, often lasting two to three weeks without chipping.

Until recently, the use of TPO was strictly regulated under cosmetic legislation. It was only permitted in professional settings, in nail preparations, and at a maximum concentration of 5%. This restriction was already intended to limit the exposure of consumers and nail technicians to TPO, while maintaining the technical efficacy of gel nail polishes. Despite these precautions, TPO has remained ubiquitous in formulations, to the point of becoming one of the key ingredients in gel nail polishes. It is precisely this dependence of manufacturers on this molecule that makes its ban all the more significant for the sector.

In May 2025, the European regulation known as Omnibus VII introduced a new series of restrictions on cosmetic products. Among these was the ban on TPO, now classified as CMR category 1B, meaning toxic to reproduction. In practical terms, this classification signifies that TPO is suspected of altering fertility or of having harmful effects on embryonic and foetal development. Since 1 September 2025, it has therefore been prohibited to place cosmetic products containing TPO on the market, as well as to distribute or use them in a professional service context. The measure applies immediately, obliging nail technicians to promptly replenish their stocks.

What is the European Omnibus Regulation ? This regulation updates the list of permitted or prohibited substances in cosmetics every year. The seventh instalment, published in May 2025, bans TPO and other active ingredients considered to be of concern, ensuring that the legislation is continually updated.

This ban is part of a wider framework of health vigilance concerning manicure products. As early as 2023, the French National Academy of Medicine had warned of the potential risks linked to the use of UV and LED lamps, essential for the curing of semi-permanent nail varnishes. The ban on TPO therefore does not address all the safety issues associated with semi-permanent varnishes, but it illustrates the growing trend to regularly re-evaluate cosmetic substances in light of new scientific data.

The prohibition of TPO is based on a body of toxicological studies demonstrating a risk to reproduction and embryonic development. In rats, a toxicity study showed that oral exposure to TPO at doses of 500 mg/kg/day induced a significant reduction in maternal weight gain, accompanied by foetal abnormalities, notably flexed hind limbs and incomplete ossification. In a supplementary one-generation reproductive study, males exposed from 200 mg/kg/day exhibited reduced spermatogenesis and histological alterations of the testes. Exposed females displayed a decreased number of viable litters. These data led to establishing a no observed adverse effect level (NOAEL) of 60 mg/kg/day and to classifying TPO as a reproductive toxicant.

Although interesting, this study demonstrates oral toxicity of TPO, not percutaneous toxicity. Furthermore, the doses administered to the rats were very high, well above the maximum concentration used in cosmetics, and thus potentially unrepresentative.

Regarding the genotoxicity and carcinogenic potential of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, the findings are more nuanced. Several batteries of tests in vitro have shown no direct mutagenicity of TPO. Among these is the Ames test, which evaluates a substance’s ability to induce mutations in various strains of Salmonella typhimurium ; the chromosomal aberration assays in hamster cells, which detect structural changes in chromosomes; and the mutation assays on murine fibroblasts, which assess whether a compound can cause gene mutations in mouse cells. These results suggest that the molecule does not behave like a classical mutagen, capable of directly inducing DNA alterations.

Other studies conducted on human cell lines have demonstrated a dose-dependent cytotoxicity. For example, in cultures of human lung fibroblasts, exposure to concentrations of 25 to 50 µM resulted in a marked reduction in cell viability and an increase in DNA fragmentation, indicative of apoptosis.

However, these various studies have been carried out in vitro or relate to the effects of oral administration in rats. They therefore do not allow to assess the compound’s ability to penetrate the nail, which is an especially impermeable medium. In this context, diffusion poses a dual challenge: first crossing the nail’s keratinised structure, then reaching the underlying skin. To date, it therefore remains uncertain whether TPO can effectively penetrate this barrier and subsequently enter the bloodstream.

One particularly concerning aspect relates to the phototoxic potential of TPO. As it is used in semi-permanent nail varnishes in combination with UV or LED lamps, several researchers have studied its effects under irradiation. Experiments have shown that under UV light (405 nm, thus close to the wavelengths used in salons, which range from 315 to 400 nm), TPO can release free radicals, generating significant oxidative stress. This has been demonstrated over short exposure times (between 1 and 15 minutes), corresponding to salon exposure durations (< 10 minutes). This stress activates intracellular signalling pathways, notably the JNK pathway and mitochondrial caspases, leading to cellular apoptosis. While some authors mention a potential application in experimental anticancer phototherapy, these results primarily reinforce the idea that TPO can become problematic when activated by a light source, a scenario that corresponds precisely to its use in semi-permanent nail varnishes.

However, this phototoxicity of TPO has been demonstrated in vitro, and not in vivo, which again limits the conclusions.

With the ban on TPO, nail technicians must swiftly adapt their practices. Several manufacturers have anticipated this regulatory change and now offer reformulated semi-permanent polishes using alternative photoinitiators considered safer. Legal options include, for example, BAPO (bisacylphosphine oxide) and TPO-L, a modified, less toxic version of TPO, which enable curing under UV or LED lamps without exposing users to the identified risks of TPO. It is also recommended that consumers check the product composition used in salons and ensure that TPO (INCI: Trimethylbenzoyl Diphenylphosphine Oxide) is not listed among the ingredients.

Finally, it should be borne in mind that safety is not confined to the choice of photoinitiator. The use of UV/LED lamps must be properly regulated, and exposure times should be limited.

• NARAYAN R. J. & al. The photoinitiator lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate with exposure to 405 nm light is cytotoxic to mammalian cells but not mutagenic in bacterial reverse mutation assays. Polymers (2020).

• XIAO P. & al. Photoinduced free radical‑releasing systems and their anticancer properties. Photochemical & Photobiological Sciences (2022).

• KWON J.-S. & al. Cytotoxicity, colour stability and dimensional accuracy of 3D printing resin with three different photoinitiators. Polymers (2022).

• Scientific Committee on Consumer Safety (SCCS). Safety assessment of trimethylbenzoyl diphenylphosphine oxide as used in cosmetics. Cosmetic Ingredient Review (2025).

• Règlement UE n°2025/877 du Parlement européen et du Conseil. Journal officiel de l'Union Européenne (2025).