GR410 states:
“For purposes of this rule, a prohibited substance is:
a. Any stimulant, depressant, tranquilizer, local anesthetic, psychotropic (mood and/or behavior altering)
**substance, or drug which might affect the performance of a horse and/or pony (stimulants and/or depressants are defined as substances which stimulate or depress the cardiovascular, respiratory or central nervous systems), or any metabolite and/or analogue of any such substance or drug, except as expressly permitted by this rule.”
According to Dr Google:
L-theanine is an amino acid known for its calming effects. It’s said to help ease anxiety, stress, and reduce insomnia.
Therefore according to the intent of the rule which is any substance that might affect the performance of a horse, L-theanine would be illegal since it is known for its calming effects. May not be on a banned substance list, but the intent of the rule is why the rule was written in the first place.
But whatever you say - the rule book is only a guideline for those persons who like to spend their money on calming substances instead of learning to ride or buying a horse within their capabilities.