That's a slippery slope argument, that's not particularly convincing, in particular given that experimentation on humans with informed consent is very much a routine thing to do, both in psychological and in medical research.
All new drugs are tested on humans at some point, when it's not really known all that well yet what the reactions will be, that's experimentation on humans right there, and very crucial experimentation at that, as that's how we figure out what's safe and what is not. The one thing that you can not do without, though, is informed consent.
Sure, but might end up on a slippery slope if you let any random person say "hey hospital, please treat me with that experimental drug; I give my informed consent and will pay you $100k for that".
That's why we usually put the burden of proof on the person conducting the experiment that there actually was informed consent. And you don't give informed consent by stating "I give informed consent", but by demonstrating that you understand the (potential) consequences, and then consenting to what you have demonstrated to understand.
All new drugs are tested on humans at some point, when it's not really known all that well yet what the reactions will be, that's experimentation on humans right there, and very crucial experimentation at that, as that's how we figure out what's safe and what is not. The one thing that you can not do without, though, is informed consent.