The side effects of Stem Cell Therapy depend on the type of cells used, how they are collected and how treatment is delivered. For orthopaedic cell-based injections, temporary pain and swelling are among the more commonly reported effects, while the long-term safety evidence is still developing.

What Are the Common Side Effects of Stem Cell Injections?

For people considering Stem Cell Therapy for joint problems such as knee osteoarthritis, the most commonly reported side effects tend to occur around the treated joint.

A 2026 systematic review of randomised controlled trials found higher rates of injection-site pain and joint swelling among people receiving mesenchymal stem cell-based treatments. Serious complications such as infection were uncommon in the studies reviewed.

Another systematic review involving 1,924 patients found that approximately 12% experienced temporary adverse effects, most commonly pain and swelling around the injection site. These effects generally lasted for less than four weeks.

You may therefore experience temporary discomfort after an injection. However, the severity and duration can vary depending on the treatment preparation, the joint being treated and your individual response.

Some procedures also involve collecting cells or cell-containing tissue from the patient's own body before treatment. Any additional procedure used to obtain or prepare the cells can introduce its own potential risks, which your clinician should explain beforehand.

Are Serious Complications Possible?

Any invasive medical procedure carries some degree of risk, even when serious complications are uncommon.

Clinical studies of orthopaedic stem cell-based injections have generally reported relatively few major complications. However, researchers continue to emphasise that studies differ substantially in the types of cells used, processing methods, doses and follow-up periods.

This means that results from one treatment cannot automatically establish the safety of every product marketed as Stem Cell Therapy.

Long-term evidence is particularly important. Many studies follow patients for months or a few years, so researchers cannot yet answer every question about the long-term risks of all types of regenerative cell treatment.

The way cells are processed also matters. In the UK, certain cell-based products may be classed as Advanced Therapy Medicinal Products. The MHRA regulates these medicines and assesses relevant products for quality, safety and efficacy when marketing authorisation is required.

Patients should therefore ask exactly what cells are being used, where they come from and how the treatment is regulated.

Are Stem Cell Transplants the Same?

No. Stem cell or bone marrow transplantation used to treat certain cancers and blood disorders is very different from an orthopaedic injection.

Hospital-based transplantation can involve intensive chemotherapy or radiotherapy before the stem cells are given. According to the NHS, possible side effects and complications can include fatigue, nausea, diarrhoea, mouth soreness, anaemia, increased susceptibility to infections, bleeding and fertility problems.

Certain donor stem cell transplants can also cause graft-versus-host disease, where transplanted immune cells attack the patient's tissues.

These risks should not be applied directly to an injection being investigated for knee osteoarthritis. Likewise, the relatively limited side effects reported in orthopaedic injection studies should not be used to describe the risks of a stem cell transplant.

The phrase Stem Cell Therapy covers very different medical procedures, which is why understanding the exact treatment being proposed is essential.

What Should You Ask Before Treatment?

Before proceeding with any regenerative treatment, make sure you understand both the potential benefits and the possible side effects.

Ask your clinician what type of cells or cell preparation will be used, where they come from and what evidence is available for your particular condition. You should also understand how the treatment is prepared and what regulatory requirements apply.

Discuss what side effects you might experience immediately afterwards and which symptoms should prompt medical attention. Increasing redness, significant swelling, fever, worsening pain or feeling generally unwell after an injection should be reported promptly to your healthcare provider.

It is equally important to discuss alternatives. Stem cell-based treatments remain an evolving area of orthopaedic medicine, and another approach may be more appropriate depending on your diagnosis, joint damage, medical history and treatment goals.

Understand the Risks Before Choosing Stem Cell Therapy

Orthopaedic Stem Cell Therapy has most commonly been associated with temporary effects such as injection-site pain and joint swelling in clinical studies. Serious complications have been uncommon in published trials, but treatments vary considerably and further long-term research is still needed.

A specialist consultation should give you a balanced explanation of the potential benefits, limitations and risks before you decide whether treatment is right for you.

If joint pain or reduced mobility is affecting your everyday life, contact Regenesis for a specialist assessment and discuss whether Stem Cell Therapy or another regenerative orthopaedic treatment may be appropriate for your condition.