Yes, “therapeutical” is a real word. It appears in the Oxford English Dictionary with recorded usage dating back to 1606, making it older than the more common form “therapeutic,” which has citations from 1646 onward. Both are adjectives meaning the same thing: relating to the healing or treatment of disease. In modern English, though, the two words are far from equally popular.
Why “Therapeutic” Dominates Today
“Therapeutical” and “therapeutic” have coexisted for over four centuries, but English has a well-documented tendency to shed the “-ical” ending from adjectives over time. The same pattern shows up in pairs like “problematical/problematic,” “alphabetical/alphabetic,” and “symmetrical/symmetric.” In each case, the shorter form gradually becomes standard while the longer form fades into rarity. “Therapeutic” won that competition decisively, and today it is the form you’ll find in medical journals, news articles, textbooks, and everyday conversation.
If you use “therapeutical” in writing, most readers will understand what you mean, but many will assume it’s a mistake. Spell-checkers frequently flag it. Style guides don’t recommend it. It’s not wrong in the strict dictionary sense, but it sounds archaic in a way that can distract from your point.
When You Might Still See “Therapeutical”
The word occasionally surfaces in older texts, legal documents, and translations from other languages. Some European languages use forms closer to “therapeutical” (French has “thérapeutique,” for instance), and non-native English speakers sometimes carry that extra syllable into their English writing. You may also encounter it in historical medical literature from the 17th through 19th centuries, when both forms were in active use and neither had a clear edge.
The Adverb Form
If you need to turn either adjective into an adverb, the standard form is “therapeutically,” which has been in use since at least 1876. Interestingly, the OED originally published “therapeutically” as part of the entry for “therapeutical” rather than “therapeutic,” reflecting the older adjective’s role in generating the adverb. Regardless of which adjective you prefer, “therapeutically” is the only adverb form in common use. You would say “the treatment is therapeutically effective,” never “therapeuticly.”
Which One Should You Use?
For any modern writing, go with “therapeutic.” It’s shorter, universally recognized, and carries no risk of looking like a typo. The only reason to reach for “therapeutical” is if you’re deliberately echoing an older style, quoting a historical source, or writing in a context where the longer form is established convention. In all other cases, the shorter word does the same job with less friction.