Loratadine (Claritin) is a widely used over-the-counter medicine designed to relieve common allergy symptoms like sneezing, itchy or watery eyes, and runny nose. It works by targeting histamine, a natural compound the body releases during an allergic reaction. Loratadine is generally classified as a non-sedating antihistamine, meaning it is specifically formulated to provide symptom relief without causing the significant drowsiness historically associated with allergy drugs. This allows patients to maintain their daily activities.
How Loratadine Differs from Older Allergy Drugs
Loratadine is part of a newer class of medications developed as an alternative to older antihistamines like diphenhydramine. These earlier medications were effective at blocking histamine receptors, but they frequently caused significant sedation and impaired cognitive function. The development of second-generation antihistamines, which include loratadine, was driven by the need for an allergy treatment that could maintain efficacy while minimizing these unwanted side effects.
Second-generation antihistamines were designed to work primarily on peripheral receptors (those outside the brain and spinal cord). This design allows loratadine to bind to the H1-receptors responsible for allergy symptoms in the body’s tissues, like the nose and skin, without producing pronounced central nervous system effects. This distinction separates loratadine from its predecessors, making it a preferred choice for daytime use.
The Science of Low Drowsiness
The reason loratadine is considered non-drowsy lies in its limited ability to cross the Blood-Brain Barrier (BBB). Histamine receptors in the brain play an important role in regulating wakefulness, and when these receptors are blocked by a medication, it induces sleepiness. First-generation antihistamines are typically small and fat-soluble, which allows them to easily pass through the BBB and cause sedation.
Loratadine, however, is chemically structured to be less lipophilic. It is also a substrate for P-glycoprotein, a transport pump that actively removes foreign substances from the brain back into the bloodstream. Very little of the drug reaches the CNS histamine receptors that govern alertness. Since loratadine primarily blocks H1-receptors in the periphery, it successfully stops the allergic reaction without interfering with the brain’s natural sleep-wake cycle. Studies show that the effects of loratadine on sedation and cognitive performance are often comparable to those of a placebo.
When Drowsiness Might Still Occur
Despite its classification as non-sedating, some people may still experience drowsiness while taking loratadine. One common factor is taking more than the recommended daily dosage, which can increase the drug’s concentration and potentially cause sleepiness. Combining loratadine with other central nervous system depressants can heighten the risk of sedation.
Combining loratadine with other central nervous system depressants, such as alcohol, sedatives, or certain pain relievers, heightens the risk of sedation and impaired coordination. Individual differences in metabolism also play a role, as the drug is processed by specific liver enzymes. If a person is taking other medications that inhibit these liver enzymes, the concentration of loratadine in the blood may increase, raising the likelihood of experiencing drowsiness.