PART 2: What Exactly Is a Peptide?

alexfoxman
Jul 27
4 min read

Peptide molecule and research vial illustrating the difference between FDA-approved, investigational, compounded and research-use peptides.

“Peptide” is a scientific category—not a seal of safety, purity, or effectiveness.

By Alex Foxman, MD, FACP, ABOM

Double Board-Certified in Internal Medicine and Obesity Medicine

Founder, Beverly Hills Institute

“Doc, are peptides safe?”

That question sounds simple.

It is not.

Asking whether peptides are safe is like asking whether medications are safe.

Which medication? At what dose? For what condition? Manufactured by whom? And supported by what evidence?

Insulin is a peptide.

Semaglutide and tirzepatide are peptide-based medications.

So are glucagon, teriparatide, tesamorelin and many other legitimate treatments.

But BPC-157, CJC-1295 and a vial purchased online marked “research use only” also get placed under the same broad peptide label.

These are not scientifically equivalent.

What Is a Peptide?

Peptides are short chains of amino acids—the same basic building blocks used to construct proteins.

Your body naturally produces peptides that act as biological messengers. They help regulate appetite, metabolism, blood sugar, growth, inflammation, reproduction and many other functions.

Many peptides work by attaching to receptors on cells and delivering a biological instruction:

Release insulin.
Reduce appetite.
Stimulate bone formation.
Trigger or suppress a hormone.
Change an immune response.

Because peptides can target specific receptors, they can produce powerful and relatively focused effects. That precision has made them an increasingly important area of drug development. Recent scientific reviews describe approximately 100 peptide therapies approved in major markets, with hundreds more being evaluated in clinical and preclinical programs.

But “powerful” does not automatically mean “beneficial.”

A strong biological signal can create a therapeutic effect, an adverse effect—or both.

Why Are Many Peptides Injected?

Most ordinary proteins and peptides are broken apart by digestive enzymes.

That is why swallowing insulin does not work like injecting insulin. The digestive system treats it more like food than medicine.

Many peptides also have short half-lives, meaning the body breaks them down quickly. Pharmaceutical scientists may alter their structure, attach other molecules or develop specialized delivery systems to help them remain active longer.

This engineering is one reason semaglutide can be administered weekly rather than repeatedly throughout the day.

And the technology is advancing.

In July 2026, the FDA approved enlicitide, the first oral PCSK9 inhibitor for lowering LDL cholesterol. Enlicitide is a highly engineered macrocyclic peptide designed to survive oral administration—something that historically has been extremely difficult for peptide medicines.

That is real peptide innovation.

It followed laboratory development, controlled human trials, manufacturing review and FDA evaluation.

It did not begin and end with an influencer testimonial.

The Four Very Different Peptide Categories

When someone says they are “taking peptides,” I want to know which of these categories they mean.

1. FDA-Approved Peptide Medication

These medications have been evaluated for a defined formulation, dose, route and medical indication.

Examples include insulin, semaglutide, tirzepatide, teriparatide and tesamorelin.

FDA approval does not mean a drug has no risks. It means its benefits and risks have been formally studied for specific uses and its manufacturing must meet defined standards.

Approval of one peptide does not validate every peptide—or even every product claiming to contain that same molecule.

2. Legitimately Investigational Peptide

An investigational peptide may have encouraging early evidence and be studied under a formal clinical-trial protocol.

That does not make it fraudulent.

It also does not make it proven.

Investigational means researchers are still determining whether it works, what dose is appropriate and what risks may emerge.

3. Compounded Peptide

Compounding can serve an important role when a patient's needs cannot be met by an available FDA-approved product.

But compounded drugs are not FDA-approved. The FDA does not review each compounded product for safety, effectiveness or quality before it reaches patients. Problems with potency, contamination, sterility and labeling can create serious risks.

The words “made by a pharmacy” should not be confused with “FDA-approved.”

4. Internet or “Research Use Only” Peptide

This is the category that concerns me most.

In 2026, the FDA issued multiple warning letters to online peptide sellers offering injectable products while labeling them “research use only” or “not for human consumption.” The agency concluded that marketing claims showed the products were actually intended for human use and emphasized the additional danger posed by injectables that bypass the body's natural defenses against contaminants and microorganisms.

A disclaimer on a vial does not guarantee purity.

It may be a warning that the seller is trying to avoid responsibility.

The FDA Is Drawing a Sharper Line

The current FDA message is not that peptides are inherently bad.

The message is more precise:

Well-studied peptide medicines can be extraordinary. Unapproved products should not borrow credibility from approved ones simply because they share the word “peptide.”

The FDA continues to identify limited human safety information and potential concerns involving impurities, aggregation and immune reactions for frequently promoted compounds such as BPC-157, CJC-1295, injectable GHK-Cu and ipamorelin.

That does not prove every one of these compounds will ultimately fail.

It means the evidence has not yet earned the confidence of established medicine.

My Bottom Line

I am not anti-peptide.

I prescribe peptide medications when the science, indication and expected benefit justify their use.

I am also excited about where peptide engineering may take medicine next.

But the word peptide tells you what kind of molecule something is.

It does not tell you:

Whether it works
Whether it is safe
Whether humans have studied it
Whether the vial is sterile
Whether the dose is accurate
Whether the long-term risks are known

So the next time someone says:

“It is only a peptide.”

Remember:

“Natural” does not mean harmless. “Experimental” does not mean proven. And “research use only” certainly does not mean safe to inject.

Next: Part 3—BPC-157, TB-500 and the Promise of Faster Healing

These may be the most aggressively promoted recovery peptides on the internet.

What do the animal studies actually show?

What human evidence exists?

What risks remain unknown?

And would I recommend them to my own patients?

Evidence over hype. Science over social media. Patient safety first.

— Alex Foxman, MD, FACP, ABOM