Thimerosal

• BPThiomersal
• Ph. Eur.Thiomersal
• USP–NFThimerosal

Thiomersal; thiomersalate; merthiolate; sodium ethylmercurithiosalicylate; mercurothiolate; thiomersalum

Ethyl(2-mercaptobenzoato-S)mercury, sodium salt · [54-64-8]

UNII 2225PI3MOV · PubChem CID 16684434

C9H9HgNaO2S · 404.81

Antimicrobial preservative

Typical concentrations: — · Dosage forms: Sterile (parenteral/ophthalmic)

An organomercurial antimicrobial preservative used in vaccines, biological products and other injections at about 0.01% w/v, and in ophthalmic and nasal preparations, typically at 0.001–0.01% w/v. It has also been used in contact lens solutions and as a topical antiseptic, although such uses have declined sharply.

A light cream-coloured crystalline powder with a slight characteristic odour.

Mercury content

about 49.5% w/w Hg

Melting point

about 232 °C, with decomposition

pH

about 6–8 (1% w/v aqueous solution)

Solubility

very soluble in water; freely soluble in ethanol (95%); practically insoluble in ether and benzene

Antimicrobial spectrum

bacteriostatic and fungistatic; more active against Gram-positive organisms and fungi, with slow action against some Gram-negative bacteria

Sensitive to light and air; solutions degrade on exposure to light and are less stable in alkaline conditions. Solutions can be sterilised by autoclaving. Store the solid in a well-closed, light-resistant container in a cool, dry place.

Incompatible with strong acids (which may precipitate the free acid), strong bases, oxidising agents, heavy-metal salts and some metals such as aluminium. Its activity is reduced by sodium metabisulfite, thioglycolates, lecithin and proteins, and it is taken up by rubber closures and some plastics.

Prepared by reacting ethylmercuric chloride or hydroxide with thiosalicylic acid in the presence of sodium hydroxide, followed by isolation of the sodium salt.

It is metabolised to ethylmercury and thiosalicylate. Delayed-type contact hypersensitivity is relatively common, and ophthalmic use can cause conjunctival irritation. Although reviews have not shown harm from the amounts in vaccines, it was removed or reduced to trace levels in routine childhood vaccines in the USA and Europe as a precaution, and multidose presentations elsewhere still use it.

Toxic by ingestion, inhalation and skin contact because of its mercury content. Handle with gloves, eye protection and respiratory protection in a well-ventilated area, and dispose of waste as mercury-containing material.

Included in the FDA Inactive Ingredient Database (intramuscular, intravenous and subcutaneous injections; ophthalmic, otic, nasal and topical preparations) and in medicines licensed in Europe. Its use is restricted under mercury regulations in many regions, with exemptions for vaccine preservation.

Phenylmercuric acetate; phenylmercuric borate; phenylmercuric nitrate.

Because of the hypersensitivity and mercury concerns, alternative preservatives are generally preferred for new products. In ophthalmic solutions it has been combined with other preservatives to widen the spectrum of activity.