Hay fever, or allergic rhinitis, is the most common chronic disease, affecting up to 30 percent of the population. It is the most common reason for chronic sinus and nose problems. Allergic rhinitis is defined as inflammation and irritation of the nasal passages due to seasonal and year-round allergens.

An allergy is an abnormal reaction by a person's immune system against a normally harmless substance. During an allergic process, the substance responsible for causing the allergy (like cat dander or pollen) binds to allergic antibodies present on allergic cells in a person's body, including mast cells and basophils. These cells then release chemicals such as histamine and leukotrienes, resulting in allergic symptoms.
When histamine is released by allergic cells in the nose and eyes, the result is sneezing, runny nose, itchy eyes/nose/throat, nasal congestion and post-nasal drip. These are the symptoms of hay fever, also known as allergic rhinitis. 
Antihistamines are medications that block a receptor for histamine, thereby stopping the symptoms that histamine causes. Antihistamines are the most commonly used medications to treat allergic rhinitis
 

Causes of hay fever
Those at risk for the development of allergic rhinitis include people with a family history of allergy, a mother who smoked during pregnancy, and living a modernized lifestyle (urban setting, higher socioeconomic status, small family size). The presence of pets, especially multiple dogs, in the home at the time of birth appears to protect against the development of allergic diseases such as hay fever.
The above phenomenon is explained by the “hygiene hypothesis,” which suggests that since we live in a cleaner environment, our immune systems do not need to fight as many infections as in the past. We don't grow up on farms around animals, we don't play in the dirt, we receive vaccines to protect against infections, and we receive antibiotics when we do have infections. As a result, the immune system is less stimulated from an infection-fighting mode, and switches to allergy mode.
Allergic rhinitis also influences other diseases. Uncontrolled hay fever symptoms can lead to sinus infections, ear infections and worsening of asthma. And people with allergic rhinitis are more prone to illnesses, since the inflammation in the nose makes them more susceptible to the virus that causes the common cold.
 
 
Oral anti-histamines
The first generation anti-histamines, which includes Benadryl®, are generally considered too sedating for routine use. These medications have been shown to affect work performance and alter a person's ability to operate an automobile.
Newer, second-generation anti-histamines have now become first-line therapy for people with allergic rhinitis. These medications include cetirizine (Zyrtec®), fexofenadine (Allegra®), and loratadine (Claritin®). Newer formulations derived from these are ie Xyzall (levocetirizine) and Aerius (desloratadin)
These medications have the advantage of once daily dosaging, start working within a few hours, and therefore can be given on as “as needed” basis. The medications are particularly good at treating sneezing, runny nose, and itching of the nose as a result of allergic rhinitis. Side effects are rare, and include a low-rate of sedation or sleepiness, but much less than the first-generation anti-histamines.


Topical nasal steroids
This class of allergy medications is probably the most effective at treating nasal allergies. There are numerous topical nasal steroids on the market with little difference, and are all available by prescription. 
This group of medications includes fluticasone (Flixonase®), mometasone (Nasonex®), budesonide (Rhinocort Aqua®), triamcinolone (Nasacort AQ®) and beclomethasone (Beconase AQ®).
Nasal steroids are excellent at controlling allergic rhinitis symptoms. However, the sprays need to be used daily for best effect and therefore don’t work well as needed. Side effects are mild and limited to nasal irritation and nose bleeds. The use of these nasal sprays should be stopped if irritation or bleeding is persistent or severe.


Other nasal sprays
There are two other nasal sprays available, a nasal anti-histamine and a nasal anti-cholinergic. The anti-histamines, levocabastine (Livocab®) or azelastine, are effective at treating allergic and non-allergic rhinitis. It treats all nasal symptoms similar to nasal steroids, and should be used routinely for best effect. Side effects are generally mild and include local nasal irritation.
Nasal ipratropium (Atrovent nasal®) works to dry up nasal secretions, and is indicated at treating allergic rhinitis, non-allergic rhinitis and symptoms of the common cold. It works great at treating a “drippy nose”, but will not treat nasal itching or nasal congestion symptoms. Side effects are mild and typically include local nasal irritation and dryness.


Over-the-counter nasal sprays
This group includes cromolyn nasal spray and topical decongestants such as oxymetazoline (Afrin®) and xylomethazolin (Otrivin®). Cromolyn works by preventing allergic rhinitis symptoms only if used before exposure to allergic triggers. This medication therefore does not work on an as-needed basis.
Topical decongestants are helpful in treating nasal congestion. These medications should be used for limited periods of 5-7 days every 2-4 weeks; otherwise there can be a rebound/worsening of nasal congestion called rhinitis medicamentosa.
The side effects of the above are both generally mild and include local nasal irritation and bleeding, but topical decongestants should be used with caution in patients with heart or blood pressure problems.


Oral decongestants
Oral decongestants, with or without oral anti-histamines, are useful medications in the treatment of nasal congestion in people with allergic rhinitis. This class of medications includes pseudoephrine (Sudafed®), phenylephrine, and numerous combination products. Decongestant/anti-histamine combination products (such as Allegra-D®, Clarinase®) are indicated for treating allergic rhinitis in people 12 years of age and older.
This class of medication works well for occasional and as-needed use, but side effects with long-term use can include insomnia, headaches, elevated blood pressure, rapid heart rate and nervousness.


Leukotriene blockers
Montelukast (Singulair®), was originally developed for asthma approximately 10 years ago, and is now approved for the treatment of allergic rhinitis as well. Studies show that this medication is not as good at treating allergies as the oral anti-histamines, but may be better at treating nasal congestion. In addition, the combination of montelukast and an oral anti-histamine may be better at treating allergies than either medication alone.
Montelukast may be of particular benefit for people with mild asthma and allergic rhinitis, since it is indicated for both medical conditions. The medication must be taken daily for best effects, and usually takes a few days before it starts working.

What are allergy shots?
When medications fail to adequately control allergy symptoms and avoidance of the trigger is not easy or possible, an allergist may recommend immunotherapy or “allergy shots”. This treatment consists of a series of injections containing small amounts of the substances to which a person is allergic. After a course of allergy shots, 80 to 90 percent of patients have less allergy symptoms, and in many cases their allergies have completely resolved. Allergy shots can be given for allergic rhino-conjunctivitis (nose and eyes), allergic asthma and insect sting allergies. 
Unlike allergy medicines, which act only to “cover up” allergic symptoms or prevent them temporarily, allergy shots fix the underlying problem of allergies. This occurs because the body treats the injection much like a vaccine, resulting in the production of infection-fighting antibodies against the pollen, dust, mold or pet dander. The body then stops producing as much allergic antibodies against the triggers, and therefore won’t have as much, or any, allergic response when exposed to the allergens. These changes can last for many years even after stopping allergy shots.
The method of immunotherapy consists of starting at a small dose that will not cause an allergic reaction, with slowly advancing the dosage until the person becomes tolerant to large amounts of the extract. These injections are initially given once to twice a week until a maintenance, or constant dose, is achieved. This usually takes approximately 3 to 6 months. Once the maintenance dosage is reached, the allergic symptoms are largely resolved in most patients. Thereafter, the injections are given every two to four weeks. Therapy is continued for 3 to 5 years total after which the patient continues to get benefit for another 5 to 10 years or longer medication free.
 

SLIT
Sublingual immunotherapy, or SLIT, is a form of immunotherapy that involves putting drops of allergen extracts under the tongue. Many people refer to this process as “allergy drops,” and it is an alternative treatment for allergy shots.
SLIT is usually delivered one of two ways: drops (or tablets) of allergen extract are placed under the tongue, then 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 daily, or multiple times per week, over a period of years.
SLIT appears to be effective in the treatment of allergic rhinitis, most studies on SLIT do show 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.  Slit drops are in Europe available against  different allergens via Alk-Abello on doctors orders.