General Opposites:
* Lose: This is the most straightforward opposite, meaning to no longer have something.
* Forfeit: This implies losing something due to a penalty or failure.
* Surrender: This means to give up something willingly or under pressure.
Context-Specific Opposites:
* Lose (ground/control/confidence): This is appropriate when referring to losing something previously held.
* Abandon: This implies giving up something completely, often without intention to reclaim it.
* Relinquish: This means to formally give up possession or control of something.
Example:
* Regain: "She regained her health after a long illness."
* Opposite: "He lost his health due to his unhealthy lifestyle."
To choose the best opposite, consider the specific context and the meaning of "regain" in that sentence.