Interview with Joshua Kritzer

Republished with permission. This interview first appeared in MedShadow Foundation on July 21, 2026, and is reproduced here in full and unaltered under a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 license. Read it at MedShadow Foundation. MedShadow Foundation is an independent nonprofit newsroom covering medication safety and accepts no pharmaceutical funding. The photograph accompanying the original is not reproduced here.

Peptide researcher Joshua Kritzer explains why promising mouse studies and online testimonials aren’t enough to prove a treatment is safe, or effective

By Sara Talpos, MedShadow Foundation, July 21, 2026

For more than two decades, chemist Joshua Kritzer, Ph.D., has studied peptides: short chains of amino acids that, when produced by the body, help cells communicate with each other. In his laboratory at Tufts University, Dr. Kritzer designs and tests peptides with the potential to treat diseases such as Alzheimer’s and cancer. To date, the Food and Drug Administration (FDA) has approved about 50 different peptides for use as pharmaceutical drugs, including popular GLP-1s, such as Ozempic and Mounjaro.

To enter the U.S. market, these approved peptides had to pass through a series of clinical trials — large, rigorously designed human studies that assess medication for safety and efficacy. (The FDA’s clinical trial requirement for drugs can be traced back to 1962.) Now, Health and Human Services Secretary Robert F. Kennedy Jr. wants to broaden access to a dozen peptides that have not completed this vetting, but are nevertheless sold on a sprawling gray market.

In late July, an FDA advisory committee will evaluate 7 of the most popular unapproved peptides, as part of a process to determine whether specialized pharmacies should be allowed to dispense the substances to patients with a physician’s prescription. These substances are currently marketed for a range of therapeutic purposes, including better sleep, increased energy, and enhanced healing from musculoskeletal injuries

Many peptide researchers, including Dr. Kritzer, are troubled by this potential pivot. All too often, promising treatments fail when they are tested in humans, notes Dr. Kritzer. And without large, long-running clinical trials, it’s nearly impossible to know whether a substance can cause side effects like cancer, which can take years to develop. “I would never recommend anybody to use something that hasn’t been proven to be safe,” said Dr. Kritzer.

MedShadow Foundation sat down with Dr. Kritzer to better understand why researchers value clinical trials and what people should keep in mind when considering whether to try an unapproved peptide. The conversation also touched on Kennedy’s argument that peptides are like supplements and therefore, might not need to be tested as rigorously as drugs.

This interview took place over Zoom and has been edited for length and clarity.

MED: My understanding is that the study of new peptide drugs typically begins with work in test tubes or petri dishes. From there, the substance might be tested in experimental animals, such as mice. And then finally, the peptide is tested in humans. Why does research progress in this way?

JK: There’s two parts to how drugs are developed. The first is discovering new molecules and showing that they do interesting things: combat disease, promote health. Then the second part is to get them actually approved as drugs.

The first part, research and development (R&D), is where scientists generate new ideas for new peptides. This is the same for any other type of molecule. Scientists make those molecules. They test them in a test tube. Maybe they see if they bind to the target protein that they think they might bind to. They test them in cultured cells — cells that are just grown in a dish — to see whether hitting that protein in those cells has the desired effect: makes the cells use more blood sugar, or kills cancer cells in a dish, that kind of thing.

From there, you would test them in a model organism, such as a mouse, to see whether they kill cancer cells in a mouse model of cancer or control blood sugar in a mouse model of diabetes. You have that progression because we take working with animals very seriously. We don’t want to move something into animals unless it really is worth testing.

MED: Some of the peptides being evaluated this summer have not been extensively studied in humans. And yet they’re quite popular on the gray market. What should people consider if they’re weighing whether to try one of these?

JK: Some people have a misconception that a supplement is some sort of natural product and a drug is something synthetic that was produced in the lab. But in fact, many drugs are made from natural products. One of the frontline treatments for breast cancer is derived from the bark of the yew tree. There are lots and lots of drugs derived from natural sources.

An important difference between a drug and a supplement is simply whether it’s actually been proven to be safe and effective in humans. So safe: it doesn’t cause toxicity. And effective: meaning it actually treats a disease or ailment, or improves health compared to whatever the standard of care is now. That’s what the FDA checks in that long process of clinical trials. It’s very expensive and very time-consuming. That’s why we have large drug companies to bankroll those efforts. These efforts can fail, and when they do, drug companies lose a lot of money.

A way to get around that is to try to offer compounds as supplements. The entire supplement aisle, even at the nutrition store, it’s a bunch of compounds that could undergo the clinical trial process. There’s nothing stopping those companies from doing that, except it’s very expensive and time-consuming. They’d rather offer the supplements as a consumer product.

The laws on supplements are no different than the laws on ground beef, or strollers, which is, if it hurts someone, you can sue the manufacturer. And if enough people sue the manufacturer, then the government can tell the manufacturer, “you can’t sell that anymore.” Whether it’s a faulty seat belt or a plant extract, that is how consumer safety works in America.

And so, it’s very, very important that people realize that when you purchase something that is not an FDA-approved drug, what you’re purchasing is a consumer product that has not been tested for efficacy, or even safety. Your recourse would only be that if it harms you, you can then sue the manufacturer.

When you’re talking about things like peptides on the gray market, now you’re going one step further from those consumer supports, where there is not a company that is stamping its name on the side of that supplement bottle that you can even sue if it harms you.

MED: What would you say to somebody who — and this is happening — goes to PubMed, reads some studies, and thinks, “there’s some really promising research in mice here, showing this peptide helps the mouse.” How likely is it that that will translate to humans?

JK: In industry R&D and in academic discovery, we have a saying that we’ve cured cancer in mice hundreds of thousands of times. Cancer progress has come a huge distance in the last 50 years, but certainly, if curing cancer in mice meant we could use the same molecules to cure cancer in humans, there would be no cancer. I personally have molecules that have cured cancer in mice that haven’t worked in humans. The same thing translates to these peptides.

The reason why actual clinical trials still fail a lot is because we just don’t understand as much about biology as the textbooks would make you think we do. Biology is way more complicated than we know; it’s not like a computer, or iPhone, or something, where we understand exactly how each circuit works. Even a simple cell — people are discovering new things all the time about what’s happening in individual cells, not to mention our whole body, with different organs communicating with each other. There are entire textbook chapters in the biology textbooks right now that are written about discoveries that happened five years ago.

It’s very easy to read the literature and think, “Oh, scientists really know a lot about how this peptide works.” And the answer is: no, we don’t. How it works in humans is often a huge question mark. A great example for people who are into peptides is the GLP-1 receptor peptides; they have effects on the brain and in the body that are just being discovered. They do so many things that we had no idea that they were doing when they first went into humans for type two diabetes 30 years ago.

You might say that is a huge success story. But I would take that as a warning: we had no idea all the different parts of the body — the brain, the liver, the digestive system, the cardiovascular system — we had no idea what this [class of] peptide was doing.

Just reading a few papers, or hearing a few anecdotes about “this one peptide made my skin tanner. Or this other peptide made me more vigorous, or made me feel younger, and there’s 10 papers to back this up” — that is the tip of the iceberg. We probably have no idea what that peptide is doing in your body.

MED: I’d like to ask a follow-up question about anecdotal evidence in humans because you can find it all over the internet, as you mentioned, people saying they’ve used particular unapproved peptides and had benefits. In your view, how compelling is this anecdotal evidence, perhaps particularly when it comes to identifying potential side effects or long-term safety concerns?

JK: Anecdotal evidence is hard to resist because we do value our own experience. We value the experience of people we know. The biggest problem with anecdotal evidence is that it introduces confirmation bias, which is where you think an outcome is going to happen, and so it’s more likely for you to perceive that it happens.

Say you go to a restaurant, and it’s already been hyped up to you. You take the first bite. You’re already primed to want to love what you’re eating. You’re already primed to talk it up to your friends. If there’s one disappointing part of the meal, everyone’s just going to ignore it because everything else was so great. So if a doctor or a friend tells you, “Take this peptide, it’s going to have these effects,” and you take it, you’re way more likely to think that that effect is happening.

The opposite is also true: If somebody just starts taking a peptide, and they start to have an ulcer or stomach pain, they may be less likely to ascribe that side effect to the peptide because “my skin is getting better,” or “my mood is getting better, and so I’m going to ignore this other thing, or maybe it’s the coffee I’m drinking,” or something else. […] So the side effects and the toxicity are not going to be uncovered by anecdotes.

Cancer, especially, is something that happens over the medium- to long-term and requires thousands of individuals to detect. If 2 percent of the people who take a drug develop cancer [from the drug], that may happen only over five or 10 years. And that’s not going to be detected by only 10 people taking a drug.

That’s why we require more than anecdotal evidence to judge whether a drug is working and whether a drug is safe in the first place. Nobody should rely on an anecdote to put something in their body; that’s my personal advice.

MED: You touched on this a little bit earlier. Some people have argued that peptides should be regulated like supplements because they’re made of chains of amino acids, which are already present in the foods that we eat and in the body. Is this argument scientifically sound?

JK: The argument is not scientifically sound. That argument is not logically sound.

There are a lot of very, very toxic compounds that are made of peptides. The poison in deathcap mushrooms, which sickens and kills dozens of people every year who are foraging for mushrooms (side note, you should know what you’re foraging for, if you’re into that hobby), is a peptide. Super-potent. A very small amount — a dust amount of that pure compound — can kill you.

Just because something is made from simple, or natural, building blocks doesn’t make its potency and toxicity any less of a problem.

MED: What do you think will be the outcome of the federal proposal to loosen restrictions on peptides?

JK: Already, by floating the proposal, I think the government has sent a message that people should take warnings against taking gray-market substances less seriously. And I think that’s damaging. I said before that the difference between a supplement, or even a gray-market substance, and a drug is simply that the drug has been tested to be safe and effective. So I’m not going to say that I know for a fact which supplements, and which peptides, work and don’t work.

If the government wants to invest money to say, “Here are 20 peptides where there is anecdotal evidence. We would like to fund companies, academic researchers, whoever, to do the clinical trials on these specific compounds,” great. I would love my tax dollars to go to that. Like, let’s investigate and prove — let’s get the evidence, so that people can use these substances.

Without that evidence, though, encouraging people to use them is pretty ethically unsound.

I would never tell a friend or family member who is asking me, “should I use these peptides?” to use them. And the fact that the government is telling people, “well, you decide. We’re not going to tell you what to do,” I think that that’s a very damaging message to send because somebody is going to suffer medical consequences from taking a substance that wasn’t fully tested.

MED: Is there anything else that you’d like to add?

I think it’s a universal belief that there’s all this information out there [so] people should do their own research. This is definitely true. But you have to do your own research within a structure that follows logic, follows good reasoning, follows what we know about how biology works. Do your own research doesn’t mean to hand-wave away really red flags. Somebody who says “this is a natural substance, so how bad could it be?” That’s not logical, sound thinking.

You really have to be skeptical. Not just of Big Pharma, or other people who you think have a profit motive in keeping some substance away from you, but be skeptical of anecdotal claims, and people who claim, “Oh yeah, don’t worry about that, it’s going to be fine.”

References

• FDA Approvals | Peptide. (n.d.).
• Greene, J. A., & Podolsky, S. H. (2012). Reform, Regulation, and Pharmaceuticals — The Kefauver–Harris Amendments at 50. New England Journal of Medicine, 367(16), 1481–1483.

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Copyright © 2026, MedShadow Foundation Foundation. MedShadow Foundation Foundation is a registered 501(c)(3) non-profit organization. Republished by the American Peptide Society under CC BY-NC-ND 4.0. Original article.