You'd like to think we know just about everything we need to when it comes to common drugs used to treat major illnesses.

But we don't.

A new study shows that, yet again, we're just now learning something about a drug we should have known before it ever was approved in the first place.

As it turns out, dopamine agonists, commonly used to treat Parkinson's disease, can lead to pathological behavior such as compulsive gambling and hypersexuality. Now I'm not saying we should abandon our front line treatment for the symptoms of Parkinson's disease, but I am saying we should be a lot more careful about protecting patients from harmful side effects. You see, these types of meds have been around for a long time and I can't help but wonder why did it take so long to uncover this side effect?

The study, carried out at the Mayo Clinic and published in Mayo Clinic Proceedings, found that these compulsive behaviors may occur nearly 20 percent of the time in patients using therapeutic doses of the dopamine agonists.

Not only that, but because many compulsions often go unrecognized, the researchers believe the true number could be even higher.

Their study looked at 267 Parkinson's patients, 38 of whom were being treated with therapeutic doses of a dopamine agonist. Of them, six men and one woman – 18.4 percent – developed compulsive gambling, hypersexuality, or both.

The researchers also noted other compulsive activity.

It took as little as one month for those behaviors to start showing up, and they often continued for years, in part because no initial connection was made to the drug.

What's amazing is that none of the patients in the other treatment groups experienced these psychological conditions. None of them.

This isn't the first study to make the link, but it's the first to show it so clearly – and in such a high percentage of patients.

In the past, patients using these drugs have reported life-changing gambling and sexual behaviors, but no one took them very seriously. Meanwhile, some of these folks gambled away their life savings, or watch their marriages crumble, all because of an unreported side effect of a medication they were told would make them better.

Now, I hope these complaints won't fall on deaf ears anymore.

And if that's not bad enough, here's something even more disturbing to consider: Dopamine agonists are also used to treat restless leg syndrome. No one has suggested a link between that use of these meds and these compulsive behaviors, but someone needs to start asking questions soon.

The problem, once again, is the FDA's approve-first, ask-questions-later approach to drugs.