An Antimony pill is a pill

Pill

Pill or The Pill may refer to:* Pill , referring to anything small and round for a specific dose of medicine. The term is used colloquially in several ways:** A tablet or capsule which replaced dosing via pill...

 made from metallic antimony

Antimony

Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

. It was a popular remedy in the nineteenth century, and it was used to purge and revitalise the bowels. In use, it is swallowed and allowed to pass through the body, after which it is customarily recovered for reuse, giving rise to the name everlasting pill.

According to the Medico-Pharmaceutical Critic and Guide (1907), edited by William J. Robinson

William J. Robinson

William Josephus Robinson was an American physician and birth control advocate. He was Chief of the department of Genito-Urinary Diseases at Bronx Hosptial Dispensary, and editor of the American Journal of Urology and Sexology...

:
The "everlasting antimony pill" is mentioned by Stephen Maturin in the novel The Ionian Mission

The Ionian Mission

The Ionian Mission is a historical novel by Patrick O'Brian, set during the Napoleonic Wars. It is the eighth in the Aubrey-Maturin series.-Plot summary:...

by Patrick O'Brian

Patrick O'Brian

Patrick O'Brian, CBE , born Richard Patrick Russ, was an English novelist and translator, best known for his Aubrey–Maturin series of novels set in the Royal Navy during the Napoleonic Wars and centred on the friendship of English Naval Captain Jack Aubrey and the Irish–Catalan physician Stephen...

.