What You Need to Know About Sublingual Immunotherapy

Learn more about allergy drops under the tongue

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Sublingual immunotherapy, or SLIT, is a form of immunotherapy that involves putting liquid drops or a tablet of allergen extracts under the tongue. Many people refer to this process as “allergy drops,” and it is an alternative treatment for allergy shots. This form of immunotherapy has been used for years in Europe and has recently attracted increased interest in the United States.

Alternative Healthcare Worker Dropping CBD Oil on Patient's Tongue
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The immune system of the gastrointestinal tract tends to "tolerate" foreign substances, meaning that it does not respond in an overactive way to swallowed material. This makes sense; otherwise, the body would overreact to anything swallowed, including food. This results in fewer allergy symptoms when the body is exposed to the allergy source, such as airborne pollen or pet dander, in sublingual allergy drops.

FDA Approval and Off-Label Use

Most allergy drops are not approved by the Food and Drug Administration (FDA) in the United States. However, on March 1, 2017, the FDA approved Odactra to treat allergic rhinitis and conjunctivitis caused by house dust mites in people 18 to 65 years old.

In 2014, the FDA also approved three SLIT products. These products are Grastek (for grass allergy, marketed in Europe as Grazax), Oralair (for grass allergy), and Ragwitek (for ragweed allergy). The FDA-approved formulations are tablets rather than liquid drops.

Even though most allergy drops are not FDA-approved, some doctors prescribe them to patients "off-label," which is legal. It just means they may cost you, because your health insurance isn't likely to cover something that's not FDA-approved.

Multiple studies are currently being conducted for the purpose of trying to get SLIT FDA-approved in the United States, but is it likely that approval is still at least a few years away. Use caution and talk with a reputable allergist before choosing this treatment.

Questions that remain to be answered prior to SLIT receiving FDA approval:

  • What are the effective dose, dosing regimen, and timing of SLIT?
  • How can multiple allergens be given in an effective, convenient, and cost-effective way?
  • Is SLIT safe for high-risk groups?
  • What is the significance of the known side effects?

Before Taking

People with a documented allergic disease (allergic rhinitis, allergic conjunctivitis, and/or allergic asthma) from allergy testing may be considered for SLIT. Young children have been shown to tolerate and benefit from sublingual allergy drops, as have older adults.

People who are pregnant can continue SLIT during pregnancy but should not start the therapy while pregnant. People with severe asthma may need to be monitored more closely while on SLIT, since asthma symptoms can worsen with this therapy. SLIT may also be contraindicated in patients with a history of any severe systemic allergic reaction, a severe local reaction after taking any subligual allergen immunotherapy, or a history of eosinophilic esophagitis.

Most studies on sublingual allergy drops have focused on people who have one type of allergy, such as grass or dust mite allergy. People with multiple allergies, such as to molds, pollens, pet dander, and dust mites, may not be good candidates for SLIT, given the large volume of different allergen extracts that would need to be taken.


SLIT is usually delivered one of two ways: drops (or tablets) of allergen extract are placed under the tongue, and then they're either swallowed or spat out. Most studies have looked at swallowing the extract, which seems to work better.

Immunotherapy by the oral route (swallowed and not kept under the tongue for any period of time) causes too many gastrointestinal side effects (nausea, vomiting, diarrhea), and therefore is not used.

Generally, SLIT is administered either daily or multiple times per week over a period of years. Most patients are able to self-administer sublingual allergy drops. It is not yet known what the best dosing regimen is for SLIT.


SLIT appears to be effective in the treatment of allergic rhinitis, allergic conjunctivitis, and, to a lesser degree, allergic asthma. While most studies on sublingual allergy drops do show a benefit in the treatment of allergic disease, the results are somewhat inconsistent, with up to one-third of studies showing no benefit over placebo treatment.

Safety and Side Effects

Over the past 10 years, the safety of SLIT has been well documented. No serious or fatal reactions to sublingual allergy drops have been reported to date. Mild side effects, such as an itchy mouth, occur in the majority of people, and moderate side effects have been documented (in 1 in about every 12,000 doses), including:

  • Lip, mouth, and tongue irritation
  • Eye itching, redness, and swelling
  • Nausea, vomiting, abdominal cramping, and diarrhea
  • Sneezing, nasal itching, and congestion
  • Asthma symptoms
  • Urticaria and angioedema (hives and swelling)

Due to the safety of SLIT, people generally treat themselves at home and are followed in a clinic at close intervals to monitor response to treatment.

SLIT vs. Traditional Allergy Shots

Traditional allergy shots have been a proven therapy in treating allergic rhinitis, allergic conjunctivitis, allergic asthma, and venom allergy for nearly a century. Allergy shots have been FDA-approved for many years, although they need to be given in a physician’s office due to the potential for severe side effects. Therefore, they may be an inconvenience to many people.

Studies comparing the efficacy of traditional allergy shots and SLIT show that allergy shots are better at treating allergic rhinitis and asthma symptoms. In addition, immunologic markers and other tests that correlate with response to immunotherapy are seen less commonly in people receiving SLIT.

While allergy shots appear to be superior at treating allergic disease, SLIT will likely eventually become an important treatment option for people unwilling or unable to receive allergy shots.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Daniel More, MD
Daniel More, MD, is a board-certified allergist and clinical immunologist. He is an assistant clinical professor at the University of California, San Francisco School of Medicine and currently practices at Central Coast Allergy and Asthma in Salinas, California.