Yes, tonic is a Scrabble word!

Adjective

1. Of or relating to or producing normal tone or tonus in muscles or tissue

2. Employing variations in pitch to distinguish meanings of otherwise similar words

3. Used of syllables

4. Relating to or being the keynote of a major or minor scale

5. Imparting vitality and energy

Noun

1. Lime- or lemon-flavored carbonated water containing quinine

2. A sweet drink containing carbonated water and flavoring

3. (music) the first note of a diatonic scale

4. A medicine that strengthens and invigorates