The solution referred to in Sherlock Holmes's "seven-per-cent solution"

COCAINE

This stimulant was commonly used in 19th-century tonics and medicines before its dangers became widely known. Holmes injected it to combat boredom between cases.

Sherlock Holmes fans will recognize "the seven-per-cent solution" as a reference to his cocaine use—a 7% cocaine solution was his go-to stimulant in the original stories. The clue plays on this well-known detail, directly pointing to the drug itself as the answer. No wordplay here, just a straightforward nod to Conan Doyle’s writing and Holmes’s infamous habit.