Cell Therapy Safety Reports: Where to Look for Serious Events

By the Regenerative Evidence Guide Team

Where to Find Cell Therapy Safety Reports

If you’re trying to find out whether a stem cell, exosome, or other cell therapy product has caused serious harm to other patients, two U.S. government sources cover this directly: the FDA’s MedWatch adverse event reporting program, and ClinicalTrials.gov’s posted results for trials that have reported safety data. Neither source gives you a simple, single “safety score” for a product. Reading them correctly means understanding two things first: what counts as a serious event, and what number you’re comparing it against. For the broader framework behind questions like this one, see our Start Here guide.

Start With the FDA’s Warning About Unapproved Products

Before looking at individual reports, it helps to know the FDA’s baseline position. The agency has stated that many products marketed as regenerative medicine therapies—including stem cells, fat-derived (stromal vascular fraction) cells, umbilical cord blood or cord blood stem cells, amniotic fluid, Wharton’s jelly, and exosomes—require FDA licensure or approval before they can be legally marketed to consumers, and that most have not received it.

The FDA has said it has received reports of the following safety concerns tied to unapproved regenerative medicine products:

• Blindness
• Tumor formation
• Neurological events
• Bacterial infections, including life-threatening blood infections
• Reactions at the site of collection and administration
• Unwanted inflammatory or immune response to the cells or therapy
• Cells that moved to another part of the body and turned into unintended tissue, or grew excessively (forming a tumor)
• Failure of the therapy to work as expected when approved treatments were available
• Cross-contamination with bacteria, viruses, or mold during processing, or lack of testing for infectious diseases such as hepatitis and HIV

The FDA has also flagged a specific point of confusion worth knowing before you search further: being listed on ClinicalTrials.gov does not mean a product is legally marketed, and a firm’s registration with the FDA does not mean the product has been approved. Some clinics have pointed to these listings to suggest compliance that doesn’t exist.

Understand the Denominator Before You Trust a Number

This is the single most important habit for reading any safety report responsibly. A number like “14 serious adverse events” tells you almost nothing on its own. You need the denominator—the total number of people who received the product or were enrolled in the study—to turn that count into a rate.

In formal clinical trial reporting, this distinction has a name: the number of participants “at risk” versus the number “affected.” The at-risk number is the total population that could have experienced the event; the affected number is how many actually did. Without both figures, a raw event count can look alarming or reassuring for the wrong reasons, depending on how many people were actually treated.

This is also where the two sources in this article differ in an important way:

• ClinicalTrials.gov results postings generally do include a denominator, because trial sponsors are required to report the total number of participants in each study arm alongside adverse event counts.
• FDA MedWatch reports generally do not. MedWatch is a voluntary reporting system—patients, caregivers, or providers submit reports when something goes wrong, rather than every recipient of a product being tracked and counted. There is no reliable total count of how many people received a given unapproved product outside a clinical trial, so there is no denominator to calculate a rate against. A MedWatch report confirms that a specific adverse event was reported to the FDA; it cannot tell you how common that event is among everyone who received the product.

Keep this difference in mind as you read any number from either source: a count from a trial’s results page can usually become a rate, while a count from MedWatch usually cannot.

Understand the Reporting Period

The second detail to check before interpreting any adverse event data is the reporting period—the specific window of time over which events were collected. For a registered clinical trial, this window is defined as part of the study’s results submission and covers a set period during and shortly after the trial. MedWatch works differently: because it is a voluntary, patient- or provider-initiated system rather than a study with a defined observation window, a report can be filed whenever someone chooses to submit one after experiencing a problem, and the database builds up over time rather than within a fixed study period.

Two practical consequences follow from this:

• A trial’s serious adverse event table reflects events during that trial’s defined period only—it says nothing about problems that emerged later, or about use of the product outside that trial.
• A gap in MedWatch reports for a specific product does not confirm the product is safe. Voluntary reporting systems are known to undercount events; the absence of reports may reflect underreporting rather than an absence of harm.

Safety Report Checklist

Use this checklist when you’re evaluating any safety claim about a specific cell therapy product or provider:

1. Identify the exact product name and type. “Stem cell therapy” is not specific enough—note whether it’s autologous (from your own cells) or allogeneic (from a donor), and what tissue source is used (bone marrow, adipose/fat tissue, umbilical cord blood, amniotic fluid, exosomes, etc.).
2. Check FDA approval status. Ask the provider directly, in writing, whether the specific product has FDA licensure or approval, or is being administered under an active Investigational New Drug (IND) application. If they can’t answer clearly, treat that as a red flag rather than a technicality.
3. Search ClinicalTrials.gov for the product or sponsor by name to see whether a registered trial exists, and if results have been posted, note the denominator (total participants) alongside any adverse event counts.
4. Check whether a listing on ClinicalTrials.gov actually means what you think it means. Per the FDA’s own guidance, registration alone does not confirm legal marketing status or FDA oversight—confirm separately whether the product is being given under an active IND.
5. Search for FDA warning letters or safety notifications naming the product, manufacturer, or clinic.
6. Note what you cannot verify. If there’s no trial registration, no posted denominator, and no independent safety data—only marketing claims or testimonials—that absence of evidence is itself important information, not a neutral gap. This is the same standard we hold ourselves to; see how we research and use sources.

If You Believe You Were Harmed: How to File a Report

If you received a treatment with stem cells, exosomes, or another product marketed as a regenerative medicine therapy and experienced a bad side effect or worsening symptoms, the FDA encourages reporting it through the MedWatch Adverse Event Reporting program. You can file online through the FDA’s reporting portal, or by calling 1-800-835-4709. You can complete the form yourself or have someone else complete it for you, and a partial report is still worth submitting.

The FDA has said the most useful information to include is:

• What happened and when the problem occurred
• The type and name of the product (a brand name, or a description such as “stem cell product” or “exosome product”)
• The manufacturer, if it differs from the provider who administered it
• Whether the product was autologous (your own cells) or allogeneic (from another person)
• The condition the product was meant to treat
• When you received the treatment
• The name of the person who administered it and the facility or clinic where you received it

Reports become part of a database the FDA uses to identify unsafe products and patterns across many patients—so an individual report has value even beyond your own situation.

What This Information Can and Can’t Tell You

Neither source in this guide can tell you whether a specific product is safe for a specific person. A MedWatch report confirms an event was reported; it cannot establish how common that event is. A clinical trial’s posted results apply to the specific population and time period of that trial, and may not reflect how a product performs in wider use, at a different clinic, or with a different preparation process. If you’re weighing a specific treatment, the safest next step is a direct conversation with your own physician or specialist, who can review your medical history and help you evaluate what these reports do and don’t tell you about your situation—not a marketing claim, a testimonial, or a registry listing on its own.

This article is for general educational purposes only and is not medical advice; see our full medical information disclaimer. It does not replace evaluation by a licensed physician. If you are experiencing a medical emergency, contact local emergency services immediately.

By Regenerative Evidence Guide Editorial Team. Primary source: U.S. Food and Drug Administration, “Important Patient and Consumer Information About Regenerative Medicine Therapies” (content current as of April 8, 2024) and “Reporting Adverse Events Related to Stem Cells, Exosomes, or Other Products Marketed as Regenerative Medicine Products” (content current as of July 22, 2020). Denominator and reporting-period concepts corroborated against ClinicalTrials.gov results-reporting documentation; the specific assigned supporting-source URL could not be rendered for direct verification and should be re-checked before release. Last updated September 26, 2026.