Antonyms for the phrase "is on the up" can be different based on the context in which it is used. However, some possible opposites could be "is on the decline," "is going down," or "is deteriorating." These words indicate the opposite of something improving or getting better. For example, if someone says, "The economy is on the up," an antonym could be "The economy is on the decline." Similarly, if someone says, "My health is on the up," an antonym could be "My health is deteriorating." Thus, antonyms of "is on the up" indicate a negative or worsening trend.