Methylparaben

Methylparaben is the most used and least restricted member of a family of preservatives that became, for about fifteen years, the most publicly distrusted ingredients in cosmetics. Very little of that distrust attached to methylparaben on its own evidence. It was inherited.

The family is the parabens: esters of p-hydroxybenzoic acid, named for the para position of the hydroxyl group on the ring.^[1] Methylparaben is the smallest of them, and the word on a label is the family's, not this ester's.

p-Hydroxybenzoic acid carries a carboxylic acid at one point of a benzene ring and a hydroxyl group directly opposite it. Attaching an alcohol to the acid end gives an ester, and the alcohol chosen names the product: methanol gives methylparaben, ethanol ethylparaben, butanol butylparaben.

Chain length governs almost everything that matters. The longer the chain, the more oil-loving the molecule, the better it kills, and the more strongly it binds to oestrogen receptors in laboratory assays. Methylparaben sits at the short end, and is the weakest of the series on both counts.^[2] This is why European regulators have treated the family member by member rather than as a bloc.

The compound was familiar to nineteenth-century organic chemistry as an unremarkable ester. Its use is much younger. In 1924 Theodor Sabalitschka reported that the esters of p-hydroxybenzoic acid were antimicrobial and proposed them as alternatives to benzoic and salicylic acid, which worked only in acid conditions. Pharmaceutical use followed within a few years, under trade names including Nipagin M.^[3]

What made them dominant in cosmetics was range and indifference. They worked on moulds and yeasts as well as bacteria, across a broad pH, at low concentrations, with no colour and almost no smell. By the later twentieth century they were in the majority of water-containing cosmetics on sale.^[3]

Methylparaben protects the water phase of an emulsion against fungal and bacterial growth, and its particular strength against moulds and yeasts, alongside Gram-positive bacteria, is the reverse of phenoxyethanol's bias.^[4] The two are often combined for exactly that reason.

It is only weakly soluble in water, and formulators frequently use its sodium salt to get enough of it into the phase where it is needed. It is also, as a rule, used alongside a longer-chain paraben, because methylparaben by itself is not a strong enough antimicrobial for most formulations.

In 2004 Philippa Darbre and colleagues reported that parabens could be extracted intact from twenty human breast tumour samples.^[5] That paper is the origin of most public anxiety about the family. Its own authors stated that it could identify neither the route of entry nor the source, and it included no healthy-tissue comparison that would permit a causal reading. A larger study from the same laboratory in 2012 measured five esters at four locations across the breast in tissue from forty mastectomies for primary breast cancer, and found at least one of them in 158 of 160 samples — including in the seven patients who reported never having used an underarm cosmetic in their lives, which is the route of entry most often assumed. That paper too states that the source of the paraben cannot be identified.^[6] That parabens can be detected in human tissue is sound, and the detection is close to universal. That they cause tumours does not follow from it, and has not been established since.

European regulators acted on the longer esters. Commission Regulation (EU) No 358/2014 prohibited isopropylparaben, isobutylparaben, phenylparaben, benzylparaben and pentylparaben in cosmetics.^[7] Commission Regulation (EU) No 1004/2014 capped butylparaben and propylparaben at 0.14 per cent as acid for the sum of the two, and barred them from leave-on products for the nappy area of children under three.^[8]

Methylparaben was restricted by neither instrument. In the European Union it remains permitted under entry 12 of Annex V to Regulation (EC) No 1223/2009, the entry covering 4-hydroxybenzoic acid and its salts and esters, at 0.4 per cent as acid on its own, or at 0.4 per cent within a mixture of esters whose total does not exceed 0.8 per cent as acid.^[9] The Scientific Committee on Consumer Safety reviewed the ester again and held, in a final opinion adopted by written procedure on 14 December 2023 and corrected by a corrigendum adopted on 28 February 2024, that those concentrations are safe.^[10]