A method of treating amyotrophic lateral sclerosis (ALS) is provided to extend life expectancy in a subject. The method can include administering the ketamine to a subject having ALS such that the subject has an extended life expectancy compared to life expectancy of the subject prior to being administered the ketamine. The extended life expectancy is determined by the subject having a first progression rate of ALS prior to being administered ketamine and a second progression rate of ALS after being administered ketamine. The first progression rate is faster than the second progression rate such that the progression of ALS in the subject is slowed by the ketamine.