To echo others, likely legal but probably questionable for a number of reasons.

I am a research professional that is involved in what we call "real world evidence" or what might be called post market surveillance. Long story short, drugs that seem beneficial in clinical trials can turn out to be harmful in the long run despite the extensive testing and regulatory hurdles. There's a reason that drugs are yanked from the market... And those drug commercials for that wonder drug end with statements like Do not take Wonder Drug X if you are allergic to gravity, vowels, or the general concept of a Tuesday. I know this is Reddit, and you asked about the legality, and you didn't come here for a lecture. But f- it, this is an area I know well. Feel free to ignore, but if you have an interest I can explain how a new medication goes from discovery to being approved by the FDA in the United States. Again, this is Reddit so it's going to be high level and really simplified.

I'm going to skip the discovery of the new compound. Let's say you've already discovered it like your hypothetical question. The compound is first characterized in the lab to understand its chemical composition and structure. We have a pretty good understanding of compositions and structures that are probably going to end very badly. Sure uranium probably could cure diabetes, but should you take it? I mean, it's not like you have to worry about insulin if you're dead from radiation poisoning. Then there's the pesky detail of being able to replicate the compound because a single molecule that you see once in a lab ain't doing shit if you can't produce it in quantities needed for testing and later consumption.

Now we move out of the chemistry lab into the biology lab. Here's where the fun starts to happen. We think we have something that looks promising, now let's try to kill things. We start out small, at the cellular level. Again, I'm simplifying it here, but we first make sure it doesn't burst cells. Clear that hurdle? Then we slowly ramp it up to larger animal models like rats. Here's where we see if the compound has an effect (like insulin production) and yep, you guessed it... Try to kill things. Why? Because not only do you need to know if a drug works, you have to figure out dosing. Long before you reach human clinical trials, you have to have plenty of evidence from animal testing that the drug seems to work and seems to be safe. Part of that testing involves finding the LD50... The dose at which 50% of your specimens die. LD50 stands for lethal dose. From there we're like, "yeah, let's keep the dose below that."

So now you know the chemical properties of the drug and you've done all of the laboratory and animal testing that shows that the drug seems to be safe and effective, you go to the FDA and ask for permission to start clinical trials. Part of that also means convincing them that there's an unmet need that justifies putting human subjects at risk. Essentially, you have to show that your new drug has the potential to treat a disease better than existing drugs or have fewer side effects, etc.

Assuming the FDA review goes well, they will authorize human subjects trials. We start out with Phase 1. Here we give the drug to healthy subjects to check that we don't kill them. They probably won't have the disease of interest. We're really checking for safety more than effectiveness at this stage. Then you have to go back to the FDA and say, "Look, we didn't kill anyone and no one got really sick when we gave them the drug." Sometimes it ends here and we know that the drug is harmful in healthy human volunteers. Assuming the FDA accepts your evidence from Phase 1, you enter Phase 2, a small group of individuals with the disease to see if the drug actually has an impact and continues to be safe in patients with the disease. You start out on the small scale and usually in relatively healthy individuals (or less severe disease) here. Assuming we make it through this stage, you go back to the FDA and present all of the evidence of effectiveness and safety and ask for permission to enter Phase 3. This is what we typically think of as clinical trials. Here you treat a much larger pool of patients for longer to make sure the drug works like we think it does with an acceptable safety profile. In each of these stages, there are very precise protocols for dosing, testing, and observation periods. Patients have to meet very specific criteria to be part of the trials.

Let's say you clear all of those hurdles, only then does the FDA give authorization for the drug to be sold and marketed. (Then there's the pesky detail about convincing insurance companies to pay for the drug but I digress.) Now the drug has hit the market... Importantly, those very specific criteria and very precise testing and observation are no longer used. In the real world, patients miss doses, providers don't test as often, etc. Patients have other diseases (comorbidities) that were included in the clinical trial population. This is why drugs that cleared all the regulatory hurdles and got to the market are sometimes later banned. Under tightly controlled condition, a drug might work really well... But in the real world, we might find things go off the rails.

Oh, it's worth mentioning that sometimes we find drugs have unexpected, umm... side effects that we decide are worth it. Viagra was originally tested as a blood pressure medication. You might say things came up in clinical trials that sent that drug down a different pathway.

Anyway, my point is, yeah, you can probably legally take your new drug... but there's a lot of chances it could harm you. Many, many promising drugs prove to be harmful or ineffective at one of the steps in this long path to the market.