Antonyms for "regained strength" can be expressed in several ways. "Lost power" or "weakened" may be used to describe a decline in one's physical or mental abilities. "Deteriorated," "declined," or "degenerated" may also be utilized in this context. Alternatively, "injured," "hurt," or "wounded" may be used to describe a specific physical ailment or injury that has caused a loss of strength. Other antonyms may focus on the idea of struggle or difficulty, such as "struggled," "striven," or "endured," which suggest that the recovery process was a challenging one. Whatever the opposite of the phrase "regained strength" may be, it generally suggests a decrease or loss of physical or mental resilience.