Sublingual immunotherapy (SLIT) is a method of treating allergies. The method closely resembles conventional "allergy shots." In both of these methods, small amounts of allergenic substances are administered periodically and over a long period of time, and via a route different from that in which the body ordinarily encounters them. For example, although plant pollens ordinarily cause their allergic reactions by being inhaled; in allergy shots, pollen extracts are injected under the skin, while in SLIT they are placed under the tongue. The immune system has many components, and only one of them, the IgE/eosinophil system, produces typical allergic reactions. The intended effect of these alternate routes of administration is to “train” other branches of the immune system to neutralize allergens before the IgE/eosinophil system even "notices" that they are there.
The great potential advantage of SLIT over allergy shots is that SLIT does not involve needles; this makes it less unpleasant, and also capable of being done at home rather than at a doctor’s office. The absence of needles may also explain why SLIT has long been categorized as a form of alternative rather than conventional medicine. However, since the year 2000, a growing stream of properly designed studies of SLIT has reached publication, and gradually altered the situation. The stream has lately become a flood. In the twelve months prior to the writing of this article, two dozen randomized controlled trials of SLIT have been published, enrolling a total of almost two thousand patients, and running for as long as three years. SLIT has now become a medically accepted approach.
In June 2007, researchers who were already confident that SLIT works, published a study designed to determine the optimum length of treatment. This double-blind study of 137 people with dust-mite allergies compared the effectiveness of three years of SLIT versus two years of SLIT plus one year of placebo. The results indicated that three years of SLIT was more effective than two, producing a greater and more sustained reduction in symptom frequency and severity.
In one sense, this was an unsatisfactory result, because, in fact, it failed to establish an optimum duration of treatment. Previous studies had found two years more effective than one, and one year more effective than six months. Put together, these results are consistent with the possibility that SLIT must be used forever. However, in another sense, these “dose-related” findings strengthen the case that SLIT is indeed an effective treatment. It has long been known in medical research that when larger amounts of a treatment produce superior results, the treatment is quite likely effective.
SLIT has shown benefit for many types of allergies besides reaction to dust mites.