Antihistamines are among the most commonly used medications for allergic disorders. They are frequently used to relieve symptoms such as itching, sneezing, rhinorrhea, and urticaria that occur when histamine is released during an allergic response.
Although antihistamines are often discussed as a single medication class, there are important differences between individual agents. In particular, first-generation and second-generation H1 antihistamines differ substantially in their sedative, anticholinergic, and cognitive effects. Understanding these differences is important when selecting an appropriate medication and counseling patients about safe use.
Histamine is a naturally occurring chemical mediator stored primarily within mast cells and basophils. During an allergic reaction, exposure to an allergen can trigger these cells to release histamine into surrounding tissues.
Histamine then interacts with receptors located throughout the body. Activation of histamine-1 (H1) receptors contributes to many of the familiar manifestations of an allergic response, including itching, vasodilation, increased vascular permeability, sneezing, and increased nasal secretions.
H1 antihistamines reduce the effects of histamine at H1 receptors. In pharmacologic terms, modern H1 antihistamines are more accurately described as H1-receptor inverse agonists: they stabilize the receptor in its inactive state and decrease histamine-mediated signaling.
An important distinction is that antihistamines do not stop histamine from being produced or remove histamine that has already been released. Instead, they reduce its ability to produce symptoms through H1 receptors.
Clinical Pearl: Think of H1 antihistamines primarily as medications that interfere with the effects of histamine rather than medications that prevent histamine formation.
Systemic H1 antihistamines are commonly categorized as first-generation or second-generation agents. Both groups can reduce histamine-mediated symptoms, but their ability to enter the central nervous system differs considerably.
| Feature | First Generation | Second Generation |
|---|---|---|
| Examples | Diphenhydramine, chlorpheniramine | Loratadine, desloratadine, cetirizine, levocetirizine, fexofenadine |
| Blood-brain barrier penetration | Greater | Limited |
| Sedation | Common | Generally less common |
| Anticholinergic activity | More prominent | Minimal for most agents |
| Cognitive impairment | Greater potential | Generally lower potential |
| Use in older adults | Often requires greater caution | Generally preferred when an antihistamine is indicated |
First-generation antihistamines include medications such as diphenhydramine (Benadryl) and chlorpheniramine. These medications are effective H1 blockers, but they readily cross the blood-brain barrier and interact with receptors within the central nervous system.
As a result, sedation is one of their most important adverse effects. Drowsiness can impair reaction time, attention, coordination, and the ability to perform activities requiring sustained concentration.
Patients using sedating antihistamines should therefore be cautioned about driving, operating machinery, and performing other potentially hazardous activities until they understand how the medication affects them.
First-generation antihistamines also have varying degrees of anticholinergic activity. These medications can interfere with the actions of acetylcholine at muscarinic receptors, producing effects beyond their intended antihistamine activity.
Common anticholinergic effects may include:
These effects become particularly important when antihistamines are used by patients who are already taking other medications with sedative or anticholinergic properties.
First-generation antihistamines deserve particular caution in older adults. Increased sensitivity to sedative and anticholinergic effects may contribute to confusion, impaired cognition, urinary retention, and problems with balance or coordination.
The cognitive effects are especially relevant when first-generation antihistamines are used as over-the-counter sleep aids. Although their sedative properties can produce drowsiness, sedation does not necessarily translate into better-quality sleep, and the medication’s residual effects may persist into the following day.
Safety Point: A medication being available without a prescription does not mean it is free of clinically important adverse effects. Sedating first-generation antihistamines can significantly affect cognition and alertness, particularly in older adults.
Second-generation antihistamines were developed to provide effective peripheral H1 receptor blockade while producing fewer central nervous system effects. Common examples include loratadine (Claritin), desloratadine (Clarinex), cetirizine (Zyrtec), levocetirizine (Xyzal), and fexofenadine (Allegra).
These medications cross the blood-brain barrier less readily than first-generation antihistamines. Consequently, they generally cause less sedation, less cognitive impairment, and fewer anticholinergic effects.
This makes second-generation agents particularly useful when allergy treatment is needed during the day and maintaining alertness is important.
Not necessarily. The term “nonsedating” can be misleading because individual responses differ and some second-generation antihistamines can still cause drowsiness.
Cetirizine and levocetirizine, for example, may produce somnolence in some patients despite being classified as second-generation antihistamines. Fexofenadine is generally considered among the least sedating options.
Patients starting any antihistamine should pay attention to their individual response before driving or performing activities that require full alertness.
Patients sometimes perceive medications such as diphenhydramine as being “stronger” because they produce noticeable sedation and drying of secretions. However, these effects should not automatically be interpreted as evidence of superior control of the underlying allergic disorder.
The drying effect is partly related to anticholinergic activity rather than H1 blockade itself. A second-generation agent such as loratadine may therefore produce less drying of nasal secretions while also producing substantially fewer cognitive effects.
H1 antihistamines are particularly useful when symptoms are driven substantially by histamine release. Common clinical applications include:
One important clinical distinction is the role of antihistamines during anaphylaxis. Although H1 antihistamines may help relieve certain cutaneous symptoms such as itching and hives, they do not rapidly reverse the life-threatening airway obstruction or cardiovascular compromise that can occur during anaphylaxis.
Intramuscular epinephrine is the first-line treatment for anaphylaxis. Antihistamines should be considered adjunctive therapy rather than a replacement for epinephrine when anaphylaxis is suspected.
Choosing an antihistamine should involve more than selecting a medication based on symptom relief alone. The patient’s age, symptoms, comorbidities, concurrent medications, and need to remain alert should all be considered.
| Clinical Consideration | Why It Matters |
|---|---|
| Driving or operating machinery | Sedating antihistamines may impair reaction time and alertness. |
| Older adult | Greater susceptibility to anticholinergic and cognitive adverse effects should be considered. |
| Urinary retention risk | Anticholinergic activity may worsen difficulty with urination. |
| Need for daytime symptom control | A less-sedating second-generation agent is generally more practical. |
| Concurrent sedating medications | Combined central nervous system effects may increase drowsiness and impairment. |
| Individual response | Even second-generation antihistamines can cause drowsiness in some patients. |
When counseling a patient who is beginning an antihistamine, several practical points can improve medication safety:
H1 antihistamines are effective medications for many histamine-mediated allergic symptoms, but the choice of agent can significantly influence patient safety and tolerability. First-generation antihistamines readily enter the central nervous system and have greater sedative and anticholinergic effects, whereas second-generation antihistamines generally provide allergy control with less sedation and cognitive impairment.
Understanding these pharmacologic differences allows clinicians to move beyond simply asking whether an antihistamine works and instead consider which antihistamine provides effective symptom control with the lowest risk for the individual patient.
Robertson DB, Maibach HI. Dermatologic pharmacology. In: Katzung B, ed. Katzung’s Basic and Clinical Pharmacology. 14th ed. New York, NY: McGraw-Hill Medical; 2018:1068–1086.
