Necessary background information
I have recently started going back to the gym. Myself and my friend have both been suffering from muscle soreness. I decided to take a supplement known as creatine on the day of my workout, and I found that my soreness subsided far quicker than my friend's. My friend's soreness finally stopped, a few days after he decided to take the creatine supplement.
I'm aware that soreness relief can be influenced by other factors such as diet, but I suggested to my friend that the best explanation for his soreness relief was the fact that he consumed creatine.
However this led to a backlash from my gym partner and his analogy was that "some believe lemsip cures the cold, it doesn't mean its true" and then posited that I have a belief system and that "I believe creatine helps" in the same manner as a rebuttal to my argument.
My friend decided to make the argument that just because we believe in something, it doesn't mean that it is true. He used this to argue against my idea that creatine helps muscle soreness. He stated that I only hold a belief in the helpfulness of creatine, and that this does not make what I believe true. His conclusion was that my argument (creatine is the best explanation for his soreness relief) was wrong.
I feel that my friend may be guilty of a straw man fallacy or something, but I am not too sure. Could you suggest where his reasoning has gone wrong?