Well, there's a small chance anesthesia can affect you negatively. Like all medical interventions, it can have side effects.

I don't think anyone would debate that. But if someone is going to go with a completely unverified "everyone I know" scare tactic about a common medical procedure, then I might as well respond with the fact that everyone I know hasn't had any problems with it. It would be completely different if they said they had suffered a rare side effect which only affected them. Instead, their point seems to be that it's widely dangerous, and their only evidence for that is a vague set of symptoms they experienced which, even more vaguely, "everyone" experienced "some of".

So we've departed from the realm of clinical analysis and entered the realm of hearsay and feelings. And on that level, such a claim cannot be challenged with facts or statistics. But what an irrational claim can be challenged with is equal assertions of irrational feelings toward the opposite.

In other words, I would challenge a rational claim with another rational claim, but I won't bother with that for people whose arguments aren't based on reason. In that case, it's best to throw out a hearsay claim in exchange for a hearsay claim, and save your facts for people who care.

The goal of winning an argument isn't to convince someone that they're wrong (if this person is even real or believes what they say), it's to show the flaws in their reasoning to whoever they're trying to convince.