Cheap, completely understood, and it works

That would be aspirin. I wrote about it a long time ago, the message being that drug companies invested a lot of money in (as it turns out bogus) research and marketing in order "evergreen" aspirin. Since the patent on aspirin was gone before I was born, it sells for bupkis and anybody can make it. Ergo, Pfizer and Merck couldn't make any money off of it. Hence we got the debacle with the Cox 2 inhibitors, which were supposed to do all the good of aspiring with less risk of gastrointestinal side effects.

Sadly, no, as it turns out, because aspirin reduces the risk of heart disease and ischemic strokes, whereas the Cox 2 inhibitors increase it. I'd rather have an upset, or even bleeding stomach (generally reversible if you just stop taking the aspirin) than drop dead. And I have always felt that the side effects were overblown as a result of the drug companies hyping them in order to justify their much more expensive products. I'm not a real doctor, so don't rely on that judgment -- and for sure, not everybody can take aspirin. But at low doses, it's unlikely to cause problems.

There's no money to be made from aspirin, so it's hard to fund studies. As you may have heard, these docs in the land of the brolly and bowler hat did a secondary analysis of data from trials originally intended to determine the cardiac benefits of aspirin. Turns out it is also fairly effective at preventing many cancers. Not dramatically -- about a 20% reduction in risk of death from cancer over 20 years, and only if you keep at it for all that time. 75 mg a day will do it, more is not better!

Yes, there can be side effects but it's one more thumb on the scale in favor of more people considering this. (As always, ask your doctor, and I don't mean Dr. Ruth.) By the way, although salicylic acid is famous for being found in willow bark, it and similar compounds are found in many vegetables, which may help account for the overall goodness you get from eating lots of them. Just a speculation on my part, but it looks good.

The real lesson here is that aspirin was a miracle drug 100 years ago, but we haven't studied it nearly enough because there was no money to be made from it. If the drug companies had invested in understanding more about aspirin, instead of trying desperately to find something patentable, we would have gotten this news a long time ago. But of course they never will, because the Free Market™ doesn't allocate sufficient resources to basic research that can benefit humanity. Only government can do that, because knowledge is a public good.