1. Compete with allergy-liberated histamine for H1-receptor sites
2. Occasionally, a patient may develop a tolerance to the antihistaminic effects; changing to another antihistamine is usually effective when this occurs
Drowsiness, dizziness, fatigue, and disturbed coordination
Skin rashes and anticholinergic symptoms such as: dry mouth, urine retention, blurred vision, and wheezing may occur
Side Effects of Second-Generation Antihistamines
Little to no sedating effect
Fewer anticholinergic symptoms
Antitussives
Act on the cough-control center in the medulla to suppress the cough reflex
Types of Antitussives
Nonopioid
Opioid
Combination preparations
Opioid Antitussives
Codeine
Guaifenesin
Guaifenesin 200 mg and Codeine 9 mg
Nonopioid Antitussives
Benzonatate
Dextromethorphan
Expectorants
Guaifenesin
Dextromethorphan
Mechanism of Action of Dextromethorphan
1. Reduces the viscosity of tenacious secretions
2. Acts as a nonopioid antitussive by changing a nonproductive cough to a less frequent, productive cough
3. Acts directly on the MEDULLARY cough of the brain to depress the cough reflex, but it does NOT suppress respiration
Decongestants
Sympathomimeticamines that cause local vasoconstriction, shrinking of the nasal mucous membranes and a reduction in fluid secretion (runny nose)
Mechanism of Action of Decongestants
1. Shrinks the nasal mucosa by stimulating the alpha-adrenergic receptors in the nasal membranes
2. Promotes drainage of the sinuses and improving airflow
3. The advantage of oral decongestants is that they relieve nasal congestion for a longer period
Expectorants
Loosen bronchial secretions so they can be eliminated by coughing
Guaifenesin
The most common expectorant, used for the symptomatic relief of conditions characterized by mucus in the respiratory tract with a dry, nonproductive cough
Mechanism of Action of Guaifenesin
1. Acts by enhancing the output of respiratory tract fluid
2. The increased flow of secretions decreases mucus viscosity and promotes ciliary action
3. A combination of ciliary action and coughing then expels the phlegm from the pulmonary system