What is a safe substitute for a mouthguard?

There are many: Muscle relaxants, hypnotherapy, biofeedback, physical therapy & jaw relaxation exercises are just a few.

Depends on Your Use: The best mouth guards are based on impressions of the patients teeth. They may be used differently to reduce damage from grinding (bruxism), sports injuries, snoring and sleep apnea. There are some over-the-counter mouth guards available at drug stores which are usually softened at home in hot water and fit by the user over their own teeth. These are not as precise and their effectiveness varies.