What Is Salol? Examining a Historical Antiseptic and Its Modern Uses

October 6, 2025 •

4 min read

Introduced in the late 19th century, salol, or phenyl salicylate, was once a popular intestinal antiseptic and mild analgesic before being largely replaced in human medicine. This crystalline compound was valued for its unique ability to pass through the stomach and hydrolyze in the alkaline environment of the small intestine, releasing its active components.

Quick Summary

Salol is the former pharmaceutical name for the organic compound phenyl salicylate. It was once used as an intestinal antiseptic and mild analgesic before being discontinued for human use due to toxicity concerns related to its breakdown products.

Key Points

Identity: Salol is the former pharmaceutical name for the chemical compound phenyl salicylate.
Historical Use: It was historically used as an internal antiseptic and mild analgesic, especially for intestinal issues.
Metabolism: When ingested, salol breaks down in the small intestine into salicylic acid and the toxic compound phenol.
Reason for Discontinuation: The potential toxicity of phenol, a byproduct of its metabolism, led to its discontinuation for human medical use.
Modern Applications: Today, phenyl salicylate is primarily used in industrial settings as a UV light absorber in plastics and sunscreens, and in certain varnishes and adhesives.
Veterinary Use: It still sees limited use in veterinary medicine as a disinfectant and intestinal antiseptic.
Synthesis: The compound is produced by heating salicylic acid with phenol in the presence of a catalyst.

The Discovery and Rise of Salol

Salol's introduction was a significant event in late 19th-century pharmacology. Developed as a response to the side effects of other salicylate compounds, it was independently synthesized in the 1880s by the Polish-Swiss chemist Marceli Nencki and the German chemist Richard Seifert. The pharmaceutical company Heyden subsequently marketed it under the trade name "Salol," a contraction of "SALicylate of phenOL".

The compound was heralded for its potential to deliver its therapeutic effects directly to the intestines. Its chemical structure was designed to be stable in the acidic stomach environment but would break down upon reaching the small intestine, releasing its key components where they were intended to act. Its historical therapeutic applications included treating typhoid fever, pulmonary infections, and gastrointestinal issues.

The Pharmacology of Salol

Upon oral ingestion, salol ($C{13}H{10}O_3$) is a prodrug that undergoes hydrolysis in the small intestine. Enzymes and the alkaline intestinal environment catalyze its decomposition into two primary metabolites: salicylic acid ($C_7H_6O_3$) and phenol ($C_6H_5OH$).

Salicylic Acid: This is the same active compound derived from aspirin. It provided salol with its mild analgesic (pain-relieving) and antipyretic (fever-reducing) properties.
Phenol: Known for its antiseptic properties, phenol was the intended agent for disinfecting the gut. However, it is also a toxic substance, and its release led to significant safety concerns, especially with higher doses.

This two-pronged mechanism was considered innovative at the time, allowing for a localized effect in the gut. However, the subsequent metabolism and excretion of the phenol component highlighted the risks associated with this approach.

From Medicine to Industry: Shifting Uses of Phenyl Salicylate

As modern medicine developed and safer, more effective drugs became available, salol's use in human therapy declined. The inherent risk of phenol toxicity was a major factor in its obsolescence. Today, phenyl salicylate's primary role is no longer in human pharmacology, but it remains a valuable chemical in various industrial applications. It is still used in veterinary medicine as an external disinfectant and intestinal antiseptic.

Industrial and Veterinary Applications of Salol

UV Protection: Used as an ultraviolet filter to prevent discoloration in various plastic polymers, lacquers, and varnishes.
Cosmetics: Functions as a UV absorber in some cosmetic products, particularly older formulations of sunscreen.
Polishes and Adhesives: Incorporated into waxes, polishes, and adhesives due to its useful chemical properties.
Fragrance: Used as a fragrance ingredient, though its use is limited.
Veterinary Medicine: Remains a component of some external disinfectants and intestinal antiseptics for veterinary use.
Educational Demonstrations: Used in school laboratory experiments to demonstrate how different cooling rates affect crystal size.

Salol vs. Aspirin: A Pharmacological Comparison

While both salol and aspirin are related to salicylic acid, their pharmacological profiles and safety differ significantly. The following table highlights some key distinctions:


Feature	Salol (Phenyl Salicylate)	Aspirin (Acetylsalicylic Acid)
Composition	An ester of salicylic acid and phenol.	An acetyl derivative of salicylic acid.
Mechanism	Requires intestinal hydrolysis to release salicylic acid and toxic phenol.	Acts directly and is metabolized into salicylate. Irreversibly inhibits COX to prevent platelet aggregation.
Intended Action	Primary action as an intestinal antiseptic; also a mild analgesic.	Primary action as an analgesic, antipyretic, and anti-inflammatory agent.
Toxicity Concern	The release of phenol can cause systemic toxicity.	Primarily gastrointestinal irritation and bleeding; overdose can cause salicylism.
Modern Use	Discontinued for human medicine; primarily industrial and veterinary.	A widely used over-the-counter and prescription medication.

Safety and Side Effects

In its heyday, salol was marketed as a safer alternative to other salicylates because of its delayed breakdown. However, the release of phenol posed a serious toxic risk, especially with higher or prolonged doses. The toxic effects of salol are similar to those of phenol, affecting the central nervous system and causing issues like delirium, tremors, and coma in severe cases. Due to these risks and the development of safer alternatives, salol was ultimately withdrawn from human medicine.

While the risk of systemic phenol toxicity made salol unsuitable for internal human use, modern applications as a UV stabilizer in plastics and in veterinary medicine are safe for their intended purpose.

Conclusion

Salol represents a historical stage in pharmacology, demonstrating a clever but ultimately flawed approach to drug delivery. By attempting to localize a drug's effects, chemists inadvertently created a new pathway for toxicity. While it has disappeared from human medicine, the story of what is salol reminds us of the continuous evolution in understanding drug safety and efficacy. Its modern life in industry as phenyl salicylate, a valuable UV filter and preservative, proves that while its therapeutic days are over, the chemical itself still holds significant utility.

Find out more about the history and chemistry of salicylates from reliable sources like the NIH.

Frequently Asked Questions

Salol was originally used as an intestinal antiseptic and a mild analgesic, primarily to treat conditions like typhoid fever, gastrointestinal inflammation, and certain pulmonary affections in the late 19th century.

Salol is no longer used in human medicine because its metabolism releases phenol, a toxic compound, into the body. Safer and more effective alternatives with fewer risks are now available.

Unlike aspirin, which works directly after absorption, salol is a prodrug that is inert in the stomach. It must be hydrolyzed in the intestine to release its active components, salicylic acid and phenol, the latter of which poses a significant toxic risk.

Yes, salol is the trade name for the chemical compound phenyl salicylate. It may also be referred to by its chemical formula, $C{13}H{10}O_3$.

Today, phenyl salicylate is primarily used for industrial applications, such as a UV light absorber to prevent discoloration in plastics, lacquers, and sunscreens. It is also used in veterinary medicine.

When salol is broken down, a process called hydrolysis occurs in the alkaline environment of the small intestine. This process yields salicylic acid and phenol, both of which are then absorbed by the body.

The primary concern with salol is the potential for phenol toxicity, especially with large doses. In high concentrations, phenol can affect the central nervous system, leading to symptoms like confusion, dizziness, delirium, and tremors.