Antihistamine medications reduce your body's reaction to allergens or block symptom-causing chemicals such as histamine, the Mayo Clinic reports. Some examples of antihistamine drugs include diphenhydramine, loratadine and cetirizine. These drugs are used to treat hives, watery eyes, itching, runny nose and sneezing due to the common cold or seasonal allergies.
Fatigue and Weakness
The most commonly reported side effect of antihistamine drugs is fatigue. This may diminish over time after your body has a chance to get used to the medication. You may also feel more weak than usual. Make sure that you get enough sleep, and take a midday nap if possible to help yourself feel more awake throughout the day. If fatigue becomes severe and affects your daily lifestyle, contact your physician.
Headache
As your body gets used to your medication, your headache may subside. You may treat your headache with ibuprofen or acetaminophen per your physician's consent. If your headache becomes severe, it may be a sign of something more serious. Contact your health care provider.
Abdominal Pain and Diarrhea
Eating smaller meals more frequently and avoiding spicy or fried foods may help alleviate stomach discomfort. You may be at risk for dehydration if you have diarrhea. Make sure that you drink an adequate amount of fluids during this time. If stomach pain and diarrhea worsen or are accompanied by fever, you may be suffering from gastrointestinal illness and should see a physician.
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