at once vs forthwith

at once and forthwith both are adverbs.

Word	Noun	Adjective	Verb	Adverb
at once	No	No	No	Yes
forthwith	No	No	No	Yes

As adverbs, at once and forthwith are synonyms defined as:

at once and forthwith: without delay or hesitation; with no time intervening

Other synonyms of at once include directly, immediately, instantly, like a shot, now, right away, straight off, straightaway.

at once (adverb)	forthwith (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening
simultaneously