How to politely explain myself?

[BeagleMommy]

You don't have to explain yourself.  All you need to say is something along the lines of "I decided not to do the treatment.".  If she presses, you could say "I just don't want to".

She may be looking for reasons because she has a million and one counterpoints as to why your reasons are invalid.  She can argue with "I just don't want to".

[Tea Drinker]

I think anything from "Thanks, but I've decided it's not for me" to "I've discussed this with my doctor and we've decided to stick to the standard treatment" to "I did some research, and I'm afraid you've been misled. The treatment isn't FDA-approved, it's been banned in the U.S., and here are the studies explaining why" would be acceptable, given that she brought the subject up in the first place (rather than you asking for advice), and is now trying to sell you something.

"First treatment free" means that you would have to pay for subsequent treatments, and she would almost certainly make a profit from them. You don't owe a salesperson an explanation for your "no," but by the same token, if they push you it's not rude for you to get in their face in return.

That doing so wouldn't be rude doesn't mean it's what you want to do. It might still be more stressful than you want to deal with if you otherwise like this person. She's a saleswoman here, but not only a saleswoman: there's nothing wrong with deciding to prioritize the existing social relationship.

[Mikayla]

The logical side of me agrees with PPs that you don't owe a response and it could lead to drama.

However, I've been in this situation and I did provide the info my research coughed up because, similar to this situation, her comments were factually wrong.  I also think if someone asks, it's fair to respond. 

I don't remember the exact wording, but I mentioned I thought it sounded interesting but in researching it, I discovered x, y and z. And when I ended it, I stuck something in there that stated I hoped she found this useful, but if she wanted to discuss it further, I wasn't the person to talk to (or something like that) because I'd learned what I needed to know. 

I don't always assume the person is on a mission of some sort.  She might really want to know if she missed something, although a lot depends on how she phrased it when she asked you why.