Antihistamines refer to medications used to relieve a person of symptoms of allergies such as conjunctivitis (allergic and seasonal) and other conditions such as hives and hay fever. An allergy is a reaction or immune response to allergens, harmless substances such as pollen, animal hair, or dust. The antihistamines can be taken orally (pills, capsules, liquids, and chewable tablets) or topically (eye drops). Injectable antihistamine forms also exist. Although there are suggestions that oral antihistamines can work in eye allergies, the primary drug to alleviate eye allergy symptoms are topical antihistamines. Topical antihistamines can be combined with other medications such as mast cell stabilizers (such as sodium cromoglycate). While some are available as over-the-cover drugs, others are available via prescription.

The body's way of responding to something harmful like an infection is to release a chemical like a histamine. The histamine expands the blood vessels, thereby swelling the skin, which assists in protecting the body. However, in people having allergies, the body produces histamine as a reaction towards harmless allergy agents. The effect is unpleasant symptoms such as watery eyes, swelling, itching, and redness in the eye. Antihistamines will block histamine’s release.

Antihistamines are generally safe for people. However, certain groups of people need to consult the physician before taking over-the-counter antihistamines. These include pregnant and breastfeeding women, children below two years old, those on other medications, and those with underlying medical conditions such as epilepsy, liver disease, heart disease, and kidney disease. It's advisable always to read the leaflet that accompanies these medications for safety purposes.

Examples of topical antihistamines include olopatadine, ketotifen, pheniramine, and naphazoline. 


  • Antihistamines with a drowsy effect - They include promethazine, hydroxyzine, and chlorpheniramine
  • Non-drowsy antihistamines - They include loratadine (Claritin), fexofenadine (Allegra), desloratadine (Clarinex), and cetirizine (Zyrtec)

The choice between the two depends on the individual. People need to try several options before settling on the best option. While non-drowsy antihistamines may be the best because one avoids feeling sleepy, some people may prefer them if the symptoms deny them sleep.

Administration & Dosage

The healthcare provider can determine the type and exact antihistamine dosage right for a person. Antihistamines can be taken before someone encounters an allergen. They can also be taken after an allergic reaction to reduce symptom severity.

Depending on the symptoms, response to treatment, and as per the physician’s instruction, one can take antihistamines daily to control symptoms. Since the symptoms tend to worsen in the early mornings (between 4 am-6 am), it is recommended that antihistamines be taken at bedtime. Some antihistamines work for only four to six hours, while others can last as long as 12-24 hours. The affected person should read all the instructions on the package before taking the drug.

Generally, one drop is instilled in the affected eye twice a day or as directed by the physician or package instructions. The hands are washed first before administration. The patient should not touch the dropper's tip or allow it to meet any other surface or the eye. With the head tilted back and looking upward, the person makes a pouch by pulling down the lower eyelid. S/he applies one drop in the lower eyelid after holding the dropper directly over the eye. The person then looks downward and gently closes the eyes for a minute or two. With one finger on the eye's cornea, the person applies gentle pressure to prevent the drug from draining out. During this process, the person should not rub the eye or blink.

If the doctor has allowed continued wear of contact lenses while on antihistamines, they should be taken off before drops’ instillation. The person can wait at least 10 minutes after instillation before putting back the lenses. The dropper should not be rinsed, and after each use, the dropper cap can be replaced. People on other eye medications, whether ointments or drops, should wait for at least five minutes before applying them. It is advisable to use eye drops before ointments to enable the eye drops to enter the eye.

Antihistamines prescribed by doctors should be used regularly and preferably on the same day every day. This medication should be used until one is no longer exposed to allergic substances, even after allergic symptoms are resolved.

Immediate medical attention should be sought if new symptoms emerge or if the symptoms worsen. Those using nonprescription ketotifen should see the doctor if itching worsens or extends beyond 72 hours.

Potential Side Effects & Interactions

Antihistamines' potential side effects include blurry vision, dry eyes, reduced appetite, headache, dry mouth, problems urinating, and sleepiness. Drowsiness can cause decreased coordination, such as in driving where judgment and reaction speeds are affected.

Antihistamines should not be taken with indigestion or stomach ulcer drugs, antidepressants, and cold and cough medications containing antihistamines. Alcohol should also be avoided because it increases the likelihood of being sleepy.

Symptoms of Overdose

Antihistamine overdose symptoms include blurry vision, headache, increased heart rate, agitation, increased drowsiness, nausea, vomiting, loss of balance, and confusion. Serious overdose symptoms may include coma and seizures. These symptoms often appear within six hours of taking the medications. Accidental overdose and intentional misuse of antihistamines have reportedly caused death through seizures, cardiac arrest, and respiratory distress.


Antihistamines should be kept away from heat, moisture, and direct light. After each use, the bottle should remain tightly closed. They should be stored at room temperature and not frozen. However, some antihistamine drops can be refrigerated if they burn or sting the eyes. Antihistamines should also be kept far away from children.