Antihistamines (H₁-receptor antagonists) block histamine at H₁ receptors. This reduces itching, sneezing, watery rhinorrhea, hives, and capillary leakage. They work best when taken before allergen exposure because they prevent histamine effects rather than reverse them.

• First-generation drugs cross the blood-brain barrier and also block muscarinic, alpha-adrenergic, and serotonin receptors → sedation and anticholinergic effects; some are used as antiemetics or sleep aids.
• Second-generation drugs cross into the brain far less and are more selective → little sedation at usual doses.

Decongestants (alpha₁-adrenergic agonists) constrict dilated blood vessels in the nasal mucosa, shrinking swollen tissue and opening the nasal passages. Oral drugs act systemically; topical sprays act locally but cause rebound vasodilation when overused.

Cough drugs

• Antitussives suppress the cough reflex: centrally (dextromethorphan; opioids such as codeine and hydrocodone act on the medullary cough center) or peripherally (benzonatate numbs stretch receptors in the lungs and airways).
• Expectorants (guaifenesin) are thought to increase the volume and lower the thickness of airway secretions.
• Mucolytics break down mucus: acetylcysteine and carbocisteine disrupt disulfide bonds in mucoproteins; dornase alfa breaks down DNA in purulent sputum (cystic fibrosis).

Epinephrine (adrenaline) — the drug for anaphylaxis — stimulates alpha₁ receptors (vasoconstriction, less airway edema, raised blood pressure), beta₁ receptors (stronger heartbeat), and beta₂ receptors (bronchodilation and reduced mediator release from mast cells). Antihistamines do none of these airway and circulation actions.

Anaphylaxis order: IM epinephrine first → call for help → lie flat with legs raised (or sitting up if breathing is hard) → oxygen and IV fluids → adjuncts (inhaled albuterol for wheeze, H₁ antihistamine for itching and hives). Antihistamines and corticosteroids do not replace epinephrine and do not treat airway obstruction or shock.

• First-generation antihistamines: avoid in older adults (Beers Criteria — falls, confusion), narrow-angle glaucoma, prostatic hyperplasia/urinary retention; additive CNS depression with alcohol, opioids, benzodiazepines, and sleep aids; additive anticholinergic effects with tricyclics and oxybutynin.
• Promethazine: contraindicated under 2 years; avoid IV use where possible and never give intra-arterially or subcutaneously.
• Oral decongestants: avoid with uncontrolled hypertension, coronary artery disease, dysrhythmias, hyperthyroidism, narrow-angle glaucoma, prostatic hyperplasia; contraindicated with MAO inhibitors (and within 14 days of stopping) — hypertensive crisis. Caution with diabetes.
• Dextromethorphan: avoid with MAO inhibitors; caution with SSRIs, SNRIs, linezolid (serotonin syndrome).
• Codeine and tramadol: contraindicated in children under 12 and after tonsillectomy/adenoidectomy in those under 18; avoid in breastfeeding; do not combine with benzodiazepines or alcohol.
• Benzonatate: not for children under 10 years; keep out of reach — accidental ingestion by young children has caused seizures, cardiac arrest, and death.
• Children: over-the-counter cough and cold products should not be given to children under 2 years (FDA) and are labeled not for use under 4 years. Honey may ease cough in children over 1 year — never under 1 year (infant botulism).
• Pregnancy and lactation: loratadine and cetirizine are preferred antihistamines; intranasal corticosteroids are acceptable. Avoid oral decongestants in the first trimester and in hypertension of pregnancy; pseudoephedrine can reduce breast-milk supply.
• Epinephrine: no absolute contraindication in anaphylaxis; beta-blockers may blunt the response (glucagon may be used when epinephrine fails in a client taking beta-blockers).
• Hidden duplicates: combination cold products often contain acetaminophen — count it toward the daily maximum (see Topic 31).

Listed in priority order.

1. Anaphylaxis is the priority — for hives with throat tightness, stridor, wheeze, hypotension, or vomiting after an exposure: give IM epinephrine into the mid-outer thigh without delay, call for help, position, give oxygen, and prepare IV fluids. Repeat epinephrine every 5–15 minutes if needed. Monitor for a biphasic reaction and observe as the protocol directs.
2. Airway and secretions — assess cough: do not suppress a productive cough that clears secretions; antitussives are for a dry, exhausting cough. Encourage fluids (unless restricted) to thin mucus. Give a bronchodilator before inhaled acetylcysteine and have suction ready.
3. Safety — assess sedation and fall risk after first-generation antihistamines; use bed alarms and assist with ambulation for older adults; check for urinary retention (bladder scan if the client cannot void).
4. Cardiovascular — check blood pressure and heart rate before and during decongestant therapy, especially with hypertension or heart disease.
5. Injection safety — promethazine: deep IM preferred; if IV is used, dilute and give slowly through a patent large vein; stop immediately if the client reports burning or pain at the site.
6. Medication reconciliation — list all over-the-counter products; look for duplicate acetaminophen, antihistamines, or dextromethorphan and for MAO inhibitor use.

• Choose non-sedating antihistamines (loratadine, fexofenadine, cetirizine) for daytime use. With any sedating antihistamine, do not drive or operate machinery, and avoid alcohol.
• Take antihistamines before allergen exposure; intranasal corticosteroids work best when used daily.
• Nasal decongestant sprays: no more than 3 days to avoid rebound congestion.
• Clients with high blood pressure, heart disease, glaucoma, or prostate enlargement should ask a pharmacist before using decongestants or first-generation antihistamines.
• Read every label — many cold products combine acetaminophen, an antihistamine, a decongestant, and dextromethorphan. Use one product at a time.
• Benzonatate: swallow capsules whole; store locked away from children.
• Children: no over-the-counter cough and cold medicines under 2 years (and follow the label, which says under 4); use saline drops, suction, and fluids; honey only after the first birthday; no codeine under 12.
• Seek care for cough lasting more than 3 weeks, bloody sputum, high fever, or breathlessness.
• Epinephrine auto-injector: carry two at all times; inject into the outer thigh (through clothing if needed) and hold as the device instructs; call emergency services after use; a second dose may be given after 5–15 minutes if symptoms persist; lie down with legs raised rather than standing up suddenly; check expiration dates. Epinephrine nasal spray devices are used as labeled.

• Anaphylaxis: IM epinephrine first, 0.01 mg/kg mid-outer thigh (max 0.5 mg adult, 0.3 mg child); antihistamines are adjuncts only
• First-generation antihistamines: sedation + anticholinergic effects — avoid in older adults, glaucoma, prostatic hyperplasia
• Second-generation antihistamines are preferred for daytime allergy relief
• Intranasal corticosteroids are the most effective drugs for persistent allergic rhinitis
• Oral decongestants raise BP and heart rate — never with MAO inhibitors
• Topical decongestant sprays: maximum 3 days (rebound congestion)
• Oral phenylephrine is not effective as a decongestant (FDA proposal to remove it)
• Do not suppress a productive cough; antitussives for dry cough
• Dextromethorphan + SSRI/MAOI → serotonin syndrome
• Benzonatate: swallow whole; deadly to young children
• No OTC cough and cold products under 2 years; no honey under 1 year; no codeine under 12
• Inhaled acetylcysteine → bronchospasm; give a bronchodilator first