weaken
weak-en
verb
To make or become less strong, powerful, effective, or intense.
"The illness began to weaken his immune system."
Thesaurus
Synonyms: sabotage, counteract, countermine, subvert, undermine, damp, break, soften, dampen, de-escalate, step down
Antonyms: beef up, escalate, fortify, intensify, step up, strengthen