All allergy medications have the potential to create side effects in patients, although the severity of these may differ. Side effects most commonly associated with allergy medications include:

• Drowsiness
• Fatigue
• Impaired coordination
• Dry nose, mouth and throat
• Diarrhea, constipation or abdominal discomfort
• Blurred vision
• Difficulty urinating or urine discoloration
• Headache
• Nervousness, anxiety, restlessness or insomnia
• Irritability
• Decreased appetite
• Fluid retention or weight gain
• Increased blood pressure
• Rapid pulse
• Increased sneezing
• Nose or throat irritation (e.g., burning, stinging, soreness, hoarseness)
• Nosebleeds
• Coughing
• Bad taste in the mouth
• Wheezing
• Skin rash
• Nausea
• Muscle cramps or tremors
• Dizziness or lightheadedness
• Backache
• Tingling of the hands and feet
• Nightmares (usually in children)

Potentially more serious, less common side effects include:

• Weakened immune system
• High blood sugar levels
• Temporary psychosis (a mental condition that causes people to lose touch with reality)
• Hallucinations
• Severe nausea or vomiting
• Heart palpitations (an awareness of a strong, fast, irregular, abnormal or "galloping" heartbeat)
• Chest pain
• Severe headache
• Fever or chills
• Swelling of the face, throat, tongue, lips, eyes, hands, feet, ankles, or lower legs
• Worsening of symptoms

Prolonged use of corticosteroids can lead to Cushing’s syndrome, a hormonal disorder resulting from extended exposure of the body’s tissues to high levels of cortisol. Symptoms include:

• Round or full face
• Weight gain
• Weakness
• Thirst
• Increased fat around the neck
• Increased urination
• Depression

Long-term corticosteroid use may also lead to ulcers, cataracts (a clouding of the eye's lens), osteoporosis (a disease in which the bones are weakened, increasing the risk of fractures) and easy bruising.

Prolonged use (more than three or four consecutive days) of over-the-counter nasal spray or eyedrop decongestants can cause a “rebound” of symptoms called rhinitis medicamentosa. This involves swelling of the nasal membrane and an increase in nasal obstruction. To avoid the rebound effect, patients are advised to use the smallest dose possible as infrequently as possible. This effect is not a concern with prescription medications.