When we talk about acting as a tonic, we refer to something that has a positive and invigorating effect on our mind or body. There are several synonyms that can be used to replace this phrase, such as inspire, energize, refresh, and invigorate. Each of these words embodies the idea of boosting our energy levels and revitalizing our mood. Other alternatives that can be used include rejuvenate, stimulate, fortify, and renew. These words all convey a sense of renewing strength and vigor. By using such synonyms for act as a tonic, we can convey the same meaning in a more varied tone.