I saw this yesterday in a message about story-related evidence.

Problem:
The adjective “antidotal” has nothing to do with story-related evidence.

Explanation:
The word “antidotal” is the adjectival form of the noun “antidote”, which means a medicine for counteracting a poison.

The word “anecdotal” is the adjectival form of the noun “anecdote”, which means a short account of an incident as an unpublished narrative.

Those who confuse these two adjectives probably are distracted by

1. the fact that both adjectives have “dotal” — spelled D-O-T-A-L — in them because both nouns have “dote” — spelled D-O-T-E — in them,
2. the fact that both adjectives start with A-N, and
3. the fact that both adjectives are nine letters long.

The noun “antidote” is constructed from the prefix “anti-“, which means against, and “dote”, whose origin essentially means given. In other words, the roots of “antidote” when assembled mean given against.

The noun “anecdote” is constructed from the prefix “an-“, which means not, and “ecdote”. The “ecdote” part of “anecdote” is a combination of “ec”, which means out, and “dote”, whose origin, as just noted, means given.

So the “ecdote” part of “anecdote” essentially means published. In other words, the roots of “anecdote” when assembled mean not published.

Solution:
“anecdotal evidence”