が最後|がさいご

Once something happens, an unavoidable, usually undesirable result follows and the situation cannot be reversed. It stresses that the first action is the decisive point of no return.

The pattern sounds somewhat stiff and emphatic. It is used when warning about or complaining of situations where one step leads to consequences that cannot be stopped, such as addictive habits, persistent sales pitches, or people who won't stop talking once they start. The variant たら最後 has the same meaning.

• たら最後 / たが最後N1「たが最後」is simply another form of 「が最後」. 「たら最後」also attaches to the verb's ta-form and can be swapped with 「が最後」 when expressing negative results, but 「たら最後」 leans colloquial while 「が最後」 is more literary and stronger in tone, and 「が最後」's second clause can also express a delightful result one cannot stop indulging in.
• ばそれまでだN1「ばそれまでだ」 is used at the end of a sentence to mean that once such a situation arises, prior efforts or achievements become wasted, or—with 「と言ってしまえばそれまでだ」—that the topic ends there. It follows the ba-form or なら, has a neutral register, and cannot be interchanged with 「が最後」.
• 〜たきりN3「〜たきり」 likewise takes the verb's ta-form but focuses on there being no follow-up after the action, with a neutral register; 「が最後」 emphasizes falling into an inescapable state once something is done, is more literary, and stronger in tone. When narrating events that already happened they can sometimes be interchanged, though their emphases differ.
• そばからN1「そばから」 means something happens immediately upon doing it and repeatedly, often rendering effort futile; 「が最後」 stresses that once you do it, you fall into a point-of-no-return state. 「そばから」 can attach to both dictionary-form and ta-form verbs, has a neutral register, and cannot be interchanged with 「が最後」.
• が早いかN1「が早いか」 merely emphasizes that two actions occur almost simultaneously; its second clause must be a fact that actually occurred, and it carries no sense of 'once this happens, there is no going back.' It can take dictionary-form or ta-form verbs, whereas 「が最後」 takes only the ta-form. The two cannot be interchanged.
• かねないN2「かねない」 merely expresses worry that a bad outcome may well happen; 「が最後」 asserts that once the first condition holds, the consequence is necessarily irreversible—and it is not limited to bad outcomes. 「かねない」 attaches to the masu-stem and has a neutral register, so the two cannot be interchanged.

たら最後 is a variant with the same meaning, while ばそれまでだ says that if X happens everything is over or pointless, and 〜たきり says that nothing further happened after X. そばから describes something being repeatedly undone right after it is done, が早いか stresses that Y followed X instantly, and かねない only warns that a bad result could possibly occur, whereas が最後 asserts that the result is inevitable once X occurs.