There are various synonyms for the phrase "put to sleep" which is often used to refer to euthanizing animals or administering anesthesia to humans. Some alternatives include the phrases "sedate," "administer a lethal injection," "euthanize," "put down," "anesthetize," "render unconscious," "induce a deep sleep," and "put under anesthesia." Each phrase conveys a slightly different meaning, and may be used in different contexts depending on the situation. It's important to consider the intended meaning and tone when choosing a synonym for this phrase, as it can have significant emotional impact and implications.