Synonyms:
* Verify: To prove the truth or accuracy of something.
* Validate: To make something legally or officially acceptable.
* Substantiate: To provide evidence to support a claim.
* Assert: To state something firmly and confidently.
* Guarantee: To promise or ensure something.
* Ratify: To formally approve something.
* Endorse: To give public support to something.
Antonyms:
* Deny: To refuse to admit something.
* Refute: To prove something false or wrong.
* Disprove: To show that something is not true.
* Contradict: To say something that is the opposite of what someone else has said.
* Dispute: To question or challenge something.
* Cancel: To make something void or null.
* Revoke: To officially cancel something.
The best synonym or antonym will depend on the specific context in which you are using the word "confirm."