Yes, tougher is a Scrabble word!

Adjective

1. Not given to gentleness or sentimentality

2. Very difficult; severely testing stamina or resolution

3. Physically toughened

4. Substantially made or constructed

5. Violent and lawless

6. Feeling physical discomfort or pain (`tough' is occasionally used colloquially for `bad')

7. Resistant to cutting or chewing

8. Unfortunate or hard to bear

9. Making great mental demands; hard to comprehend or solve or believe