Allergy sufferers frequently reach for steroid nasal sprays or antihistamines to get relief from the uncomfortable, and at times debilitating, allergic reactions like runny noses, rashes or itchy eyes.

However, these medications only act to relieve the symptoms and not tackle the underlying cause, which is an overreaction by the body to a usually harmless substance like pollen or animal dander. Still, for those who are usually able to avoid the allergy-causing substances, such medications work well.

For those who can't, immunotherapy, which works like vaccination, offers a better solution as it desensitises the body to an allergen. Eventually, the body no longer reacts to the allergen, thus removing the need for long-term dependency on medication.

An example of a hard-to-avoid allergen is the dust mite, which flourishes in Singapore's hot and wet tropical climate and can be found all year round. It is a microscopic bug, often found on our mattresses, that lives off dead skin cells shed by humans.

Unfortunately, the dust mite is the most common cause here of allergic rhinitis, said Dr Dawn Lim, consultant paediatrician at Kinder Clinic in Paragon Shopping Centre. Rhinitis is a condition that consists of a group of symptoms like a runny nose, itchiness and sneezing, which are caused by irritation and congestion in the nose.

And if allergic rhinitis is not dealt with, it could lead to something worse.

According to Dr Lim, half of those with allergic rhinitis show signs of asthma, while 70 per cent of asthmatic patients initially had allergic rhinitis. Up to four in 10 children have rhinitis, of which 90 per cent show sensitivity to the dust mite.

So to treat this sensitivity with immunotherapy, a solution of dust mites is introduced in increasing amounts into the body so that it builds tolerance to the allergen. The allergen doesn't cause an allergic reaction because it is not introduced via its usual pathway. For example, the dust mite usually wreaks havoc when it is inhaled.

In treatment, the mite is introduced through an injection or by putting a solution of it under the tongue. In this way, "they get an immune reaction instead of an allergic reaction".

The latter method, called sublingual immunotherapy (Slit), is relatively new in Singapore, having been introduced here about five years back. Elsewhere, it has been used since the 1980s, said Dr Lim. "As far as sublingual is concerned, the initial studies were done on substances like grass and pollen. It was only in the last five to 10 years that there were a lot of studies looking at dust mites, which is more relevant to our community."

Which is good news for children and anyone who can't bear the trauma of being regularly stuck with needles. "In Slit, you put the solution under your tongue, leave it there for two minutes, swallow and then go off."

"You can administer it by yourself at home, unlike injections where you have to go to the doctor's clinic," says Dr Lim.

The known side effects of Slit are also less severe, like itchiness or swelling under the tongue and in rare cases, a stomach upset, she added. For the injection treatment, also known as subcutaneous immunotherapy (Scit), some people can get severe reactions like rashes, swollen eyes and throat, and a drop in blood pressure in severe cases.

Slit may be cheaper in the long run as well, since patients save on frequent doctor visits, adds Dr Lim.

From the looks of it, you may think only masochists would still chose injections over Slit. Dr Lim explains otherwise. "Scit has been around for a longer time. It's effectiveness is proven. Also, there is no head-to-head study to show which one is better."

The sublingual route may be too much hassle for some, as it requires near daily commitment to the treatment. There is an initial "build-up" phase where patients have to take the allergen solution everyday for about two weeks. Subsequently, the frequency may be cut down to three times a week.

However, "patients who want to start Slit are those who have been on daily medications for a long time and therefore, taking Slit daily is not a problem for them", says Dr Lim

In comparison, those who brave the injections go for weekly shots in the initial phase. Subsequently, they only need once-a-month shots during the "maintenance" phase. They can also see an improvement in their condition a few months faster than those on Slit, says Dr Lim.

For Slit patients, it takes about six to nine months for patients to see an improvement in their allergic symptoms. After one to two years, they may no longer need drugs to control their symptoms.

The entire course of treatment, which costs about $200 a month, takes three years or more to complete.