If you’re at risk of heart disease, you may have considered taking red yeast rice supplements to lower your cholesterol. Advertised as a naturally derived way to support heart health, red yeast rice has been around for centuries in traditional Chinese medicine and now can be bought widely, from Amazon to your local CVS. American sales reportedly total close to $50 million a year.

But what, exactly, are you getting when you buy it? A new study warns that you cannot know — in particular, you cannot know how much of a key drug, monacolin K, your supplement pills contain.

Monacolin K is chemically identical to lovastatin, a statin drug that lowers cholesterol. The new findings show that the amount of monacolin K in red yeast rice supplements remains highly variable despite FDA manufacturing standards.

In fact, the study in the European Journal of Preventive Cardiology finds that depending on the brand and its recommendations for daily servings, you may end up consuming anywhere from 0.09 to 10.94 milligrams of monacolin K a day — a range of over 120-fold. And most consumers have no idea these variations exist because companies are not required to put them on the label.

Possible consequences: If you take high levels of monacolin K while on statin drugs, you could experience severe muscle pain. On the flip-side, if your bottle of red yeast rice contains virtually no monacolin K, you might waste your money and forego other medications that would actually have brought down your cholesterol.

I spoke with the study’s lead author, Dr. Pieter Cohen of Cambridge Health Alliance and Harvard Medical School. Our conversation, edited:

How would you summarize your findings?

Basically, what we found is that if you walk into common retail stores and pick up a bottle of red yeast rice, the amount of the most important active pharmaceutical in red yeast rice, called monacolin K, was widely variable. In fact, in some supplements, there was none of it, and in other supplements, there was a prescription level.

So, the basic finding is that despite the labels all providing a very specific number of milligrams of red yeast rice, consumers would have no idea how much active drug they’re getting when they pick up a red yeast rice supplement. Therefore, there’s no way to figure out if one of these red yeast rice supplements might be good for your health.

What are the implications of not knowing how much you’re taking?

Prescription drugs are only useful if you can control the amounts you take. As we know, any extremely low levels of any drug, even a potent one, is not going to cause harm, and very high levels of a drug that is safe for lower doses may cause harm. The right dose for each patient is one that’s going to give you the most benefit without exposing you to unnecessary harm.

When you go to the store and pick up red yeast rice, there’s a complete disconnect between what you’re putting in your mouth and knowledge about how much active drug is in the product. And that disconnect makes it so that you can’t use it effectively for any health benefit.

For example, someone who prefers an all-natural way of lowering their cholesterol might buy one of those with absolutely no lovastatin in it, which would likely be completely ineffective. On the other hand, someone who’s already taking medication and wants to take red yeast rice supplements in combination might buy one that has prescription level dosages of lovastatin in it. Then, they might be having trouble because they’re taking too much lovastatin.

And one thing just to emphasize is that we’ve known that red yeast rice has unpredictable levels of monacolin K for years. We’re updating it in the setting that the FDA has been trying to improve the quality of supplements over the last decade by implementing good manufacturing practices, basically new rules and regulations about how companies need to create their supplements. And despite a lot of work by the FDA, there is no difference in terms of the variable and unpredictable quality of how much active drugs these products have in them.

Then why do these variations still exist?

One important factor is what the FDA decided was adequate in terms of supplement quality. They set the bar so low that it doesn’t help consumers. The FDA could have standardized the fermentation process according to the traditional Chinese approach so consumers would know what they’re getting. Instead, the FDA said you could make red yeast rice any way you want to as long as you write down your recipe you’re using and you repeat it every time you make a new batch. So they left the door wide open for companies to innovate any way they want to, which sounds great, but it completely disconnects what a consumer would think when reading the label — 'Oh, I'm using a traditional remedy to help my heart' — to what is actually in the pills. It's possible that some companies might simply use the least expensive formulations of red yeast rice when creating their product.

Is it possible that too much monacolin K can cause harm to some consumers?

It's possible, but it’s not going to be too common. That’s because although they’re a drug, statins are actually quite safe. If a consumer knew to stop taking the medicine if they start having severe muscle pain, they would probably be able to avoid any serious consequences of taking these drugs.

Unfortunately, that raises another issue about the regulatory framework for supplements. We know monacolin K can cause muscle breakdown, and that can be dangerous. So it would be pretty simple to put a warning on labels saying, 'If you start having severe muscle pains, please stop using this product.' Or the FDA could require known adverse effects of a supplement to be listed on the label. But given the current regulatory framework, consumers cannot get useful information about what they’re purchasing, even in the most mainstream stores.