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Lemtrada Side Effects

Side effects of Lemtrada range from rash, headache and fever to thyroid disorders, infections, and serious, sometimes fatal autoimmune conditions. In November 2018, the FDA added its strongest warning to the drug’s label about stroke and tears in arteries in the head and neck.

Last Modified: September 5, 2023
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From 2014 to September 30, 2019, Americans reported 4,252 cases of adverse events related to Lemtrada. Of those, 3,118 were serious, including 139 deaths, according to the FDA Adverse Events Reporting System (FAERS).

Because of the possibility for serious side effects, the drug is only available through a restricted distribution program called the Lemtrada Risk Evaluation and Mitigation Strategy (REMS) Program.

Lemtrada is available only through the Lemtrada Risk Evaluation and Mitigation Strategy (REMS) Program, a restricted distribution program to help ensure that the medicine’s benefits outweigh the risks.

On Nov. 29, 2018, the U.S. Food and Drug Administration warned about rare but serious risks of stroke and tears in blood vessel walls with Lemtrada.

Lawyers are filing lawsuits against Lemtrada’s manufacturer, Sanofi Genzyme, on behalf of people who suffered ischemic stroke, hemorrhagic stroke, head or neck (cervicocephalic) arterial dissection, or death due to these complications.

Rash, Headache and Other Common Side Effects

Common side effects of Lemtrada can happen immediately after taking the drug. They are typically less serious and may go away after a short time.

In controlled clinical trials, researchers studied the drug in 811 patients with relapsing forms of MS and compared it to treatment with interferon beta-1a. The most common side effects occurred in at least 10 percent of participants and happened more often in the group that had received Lemtrada.

Common side effects include:
  • Abdominal pain (stomach pain)
  • Arthralgia (joint pain)
  • Back pain
  • Diarrhea
  • Dizziness
  • Fatigue
  • Flushing (temporary reddening of the skin)
  • Fungal infection
  • Headache
  • Herpes viral infection
  • Insomnia (difficulty sleeping)
  • Nasopharyngitis (cold symptoms)
  • Nausea
  • Oropharyngeal pain (mouth and throat pain)
  • Pain In extremity
  • Paresthesia (numbness or tingling)
  • Pruritus (itching)
  • Pyrexia (fever)
  • Rash
  • Sinusitis (sinus inflammation)
  • Thyroid gland disorders
  • Upper respiratory tract infection
  • Urinary tract infection
  • Urticaria (rash)
  • Vomiting

Sanofi Genzyme and the FDA gathered more data about postmarketing side effects of Lemtrada. Postmarketing reports come from people who used the drug after it hit the market. These reports are voluntary, and there is no way to verify how many people may have suffered these reactions or if the drug caused them.

Reported postmarketing side effects include stroke, infections, autoimmune hepatitis, and disorders of the gastrointestinal system, pulmonary system, blood and lymphatic system and immune system.

Stroke and Blood Vessel Tears

In November 2018, the FDA released a safety communication warning about a rare but serous risk of stroke and tears in the lining of the arteries in the head and neck with Lemtrada. These tears are called arterial dissections. These conditions can lead to permanent disability or death.

Most of the people who suffered these complications developed symptoms within one day of receiving the drug. One patient had symptoms three days after treatment.

Infographic about artery dissection
Lemtrada can cause tears called dissections in the arteries of the neck and head.

According to the November 2018 FDA Safety communication, worldwide, there have been 13 cases of ischemic and hemorrhagic stroke or arterial dissection that occurred shortly after a patient received Lemtrada. Twelve of these cases happened within one day of receiving the drug.

The agency required Sanofi Genzyme to include the risk of stroke in the drug’s black box warning.

The European Medicines Agency (EMA) is conducting a review of Lemtrada after it received reports of problems with the heart and blood vessels occurring within one to three days of treatment. These include bleeding in the lungs, heart attack, stroke and arterial dissection in the neck and head.


Lemtrada’s prescribing information has a black box warning for autoimmunity. This means the drug can cause autoimmune disorders. Some can be serious and life-threatening. They include kidney, liver and thyroid problems, and immune thrombocytopenic purpura, a condition in which the body attacks platelets. Patients who receive Lemtrada will have regularly scheduled laboratory tests that measure blood counts and check the function of the kidneys, liver and thyroid. These tests can help check for signs of autoimmune conditions.

People who take the drug have an almost 50 percent risk of developing an autoimmune disorder within 5 to 7 years of treatment, according to researchers from Queen Mary University of London. The researchers based their findings on unpublished data from Lemtrada’s phase III clinical trials.

People who take Lemtrada have close to a 50 percent risk of developing an autoimmune disorder that may be serious or fatal.

The researchers discovered that when Lemtrada removes B and T immune cells to reduce MS symptoms, the B cells repopulate quickly while T cells do not. They theorize this causes the autoimmune responses.

Thyroid Disorders

In clinical trials, 36.8 percent of people treated with Lemtrada suffered thyroid disorders, according to the drug’s label. Newly diagnosed thyroid disorders continued to occur more than seven years after the first dose.

Of these thyroid disorders, about 5.2 percent were serious and included psychiatric and cardiac events. Some people required surgery to remove the damaged thyroid.

Inflamed thyroid gland
About 37 percent of people in Lemtrada clinical trials suffered a thyroid problem.

These thyroid disorders are especially dangerous to pregnant mothers. Thyroid antibodies can pass to the fetus. One baby developed Grave’s disease after its mother took Lemtrada while pregnant.

The function of thyroid is checked every 3 months to monitor for changes. Several treatment options are available if changes do occur.

Immune Thrombocytopenic Purpura (ITP)

About 2 percent of patients who took Lemtrada suffered immune thrombocytopenic purpura, or ITP. This is a disorder in which the immune system attacks and destroys platelets.

Low platelet counts can lead to severe bleeding that may cause life-threatening problems. Bleeding may occur in the brain. The drug can also cause low red and white blood cell counts.

Blood counts are checked every month to monitor for changes. Several treatment options are available if changes do occur.

Infusion Reactions

Infusion reactions are common and potentially serious side effects of Lemtrada. Only people trained to handle these reactions can administer the drug.

In clinical trials, 92 percent of people who received Lemtrada experienced a reaction, but only 3 percent of the reactions were serious. Mild reactions included nausea, hives, itchy skin, difficulty sleeping, chills, temporary reddening of the skin, fatigue, difficulty breathing, lung changes, reduced ability to taste, indigestion, dizziness and pain.

These reactions typically happen during the infusion, but some can happen more than 24 hours after infusion.

Serious reactions in clinical trials included:
  • Anaphylaxis, including anaphylactic shock, in two patients
  • Angioedema (swelling of the skin)
  • Bronchospasm (narrowing of the airways)
  • Hypotension (low blood pressure)
  • Chest pain
  • Bradycardia (low heart rate)
  • Tachycardia (fast heart rate), including atrial fibrillation
  • Transient neurologic symptoms (short-term changes in brain or spinal cord function)
  • Hypertension (high blood pressure)
  • Headache
  • Pyrexia (fever)
  • Rash

In order to prevent or lessen the severity of infusion reactions, health care providers administer corticosteroids at least three days prior to Lemtrada treatment. They may also administer antihistamines and anti-fever medications. Additionally, the infusion of Lemtrada can be slowed to reduce the side effects. Patients are monitored closely for side effects during the infusion and for at least two hours after the infusion.

Data from one 2021 study on infusion reactions and serious Lemtrada side effects suggested that “serious IARs are more frequent in a real-world setting in contrast to clinical studies, possibly reflecting higher comorbidity in real-world patients.”


Lemtrada may cause an increased risk of cancer, including thyroid cancer and melanoma, according to the drug’s black box warning. The warning advises patients to have a skin exam before treatment to get a baseline and to continue exams yearly after that.

Thyroid Cancer

According to clinical study results reported by Sanofi on the medication insert, three out of 919 patients treated with Lemtrada developed thyroid cancer compared to none in the group treated with interferon beta-1a. In uncontrolled studies, two more Lemtrada-treated patients developed thyroid cancer.

The medication insert tells doctors and patients to look for signs of thyroid issues, such as a new lump or swelling in the neck and pain in the front of the neck.


In studies, five out of 1,486 patients treated with Lemtrada developed melanoma, according to the medication insert. One of those patients had advanced cancer.

Patients receiving the drug should have annual skin examinations.

Other Cancer Data

A 2017 review published in Therapeutics and Clinical Risk Management by Dr. Cristina Guarnera and colleagues reports the frequency of cancer occurrence observed not only in clinical trials but also used information from postmarketing surveillance and case reports.

Out of 1,486 patients treated with alemtuzumab, 29 developed malignancies.

  • Eight were thyroid cancers (six of these were stage 1 that developed 10 to 41 months after the last infusion).
  • Six were basal cell carcinomas.
  • Five were breast cancers.
  • Six were melanomas.
  • One patient died from non-EBV-associated Burkitt’s lymphoma.
  • Three patients developed Castleman disease.

In a follow-up study, 10 out of 100 patients developed precancerous or cancerous lesions. The authors concluded that data from clinical studies do not show a significantly higher frequency of cancer with Lemtrada compared to interferon beta-1a but that patients should be monitored closely even after Lemtrada treatment has stopped.

Please seek the advice of a medical professional before making health care decisions.