Allegra (fexofenadine) lasts 12 or 24 hours depending on the dose you take. The 60 mg tablet provides 12 hours of relief and is taken twice daily, while the 180 mg tablet covers a full 24 hours with a single dose. In practice, though, the strength of its effect tapers as the hours go on, and several factors can shorten or extend how long you actually feel relief.
How Quickly Allegra Starts Working
Allegra begins relieving allergy symptoms after the first dose, with noticeable improvement within one to two hours. The drug reaches its peak concentration in your bloodstream about 2.6 hours after you swallow a tablet. The liquid suspension absorbs a bit faster, peaking closer to one hour. Clinical trials showed that symptom scores dropped significantly after a single 60 mg dose, and the effect held through the full 12-hour dosing window.
When the Effect Starts to Fade
Allegra’s elimination half-life is 14.4 hours, meaning it takes that long for your body to clear half the drug. That’s why a 180 mg dose can maintain some activity across 24 hours. But “some activity” and “full relief” aren’t the same thing.
A head-to-head study measuring how well antihistamines suppressed histamine reactions in the skin found that fexofenadine’s strong suppression (at least 70% inhibition) lasted about 9 hours for the 180 mg dose and 8.5 hours for the 120 mg dose. By the 24-hour mark, fexofenadine was blocking less than 40% of the skin reaction. So while the labeled duration is 24 hours, the most robust relief happens in roughly the first 9 to 10 hours, with a gradual decline after that.
If you notice your symptoms creeping back in the evening after a morning dose, this tapering effect is the likely explanation. Some people find the twice-daily 60 mg tablets provide more consistent coverage because you’re essentially resetting the clock halfway through the day.
How Allegra Compares to Zyrtec and Claritin
In the same skin-reactivity study, cetirizine (Zyrtec) maintained strong suppression for about 19 hours, roughly double fexofenadine’s duration. Cetirizine also showed more consistent peak effects: every participant in the study achieved near-total suppression of the histamine reaction, compared to about 80% of participants on fexofenadine 180 mg.
The tradeoff is drowsiness. Fexofenadine is the least sedating of the major over-the-counter antihistamines. Cetirizine causes noticeable drowsiness in a meaningful percentage of users. If staying alert matters more than squeezing out every last hour of coverage, Allegra is often the better fit. If duration and potency matter more and drowsiness isn’t a concern, cetirizine has the edge.
Fruit Juice Cuts Its Effectiveness
One of the most common ways people accidentally shorten Allegra’s useful duration is by taking it with fruit juice. The FDA specifically warns that grapefruit, orange, and apple juice reduce how much fexofenadine reaches your bloodstream. These juices interfere with a transport protein in your intestines that helps absorb the drug. Less drug absorbed means a lower peak and a shorter window of meaningful relief.
Take Allegra with water. If you drink juice regularly, space it at least a couple of hours away from your dose.
Factors That Make Allegra Last Longer
Kidney function is the biggest variable. Your kidneys handle a large share of fexofenadine clearance. In people with reduced kidney function, the half-life extends by 31 to 72 percent. That means the drug lingers longer and its effects stretch further, but so does the potential for accumulation with repeated doses. People with kidney impairment are typically started at a lower dose of 60 mg once daily rather than the standard 180 mg.
Dosing by Age Group
Adults and children 12 and older take either 180 mg once daily or 60 mg twice daily for hay fever. For hives, the standard adult dose is 180 mg once daily. Children aged 6 to 11 typically take 30 mg twice daily, with doses spaced 10 to 12 hours apart. Each of these schedules is designed to keep drug levels in the effective range throughout the day, so sticking to the timing matters more than people realize. Taking a dose late doesn’t just delay relief; it creates a gap where symptoms can break through.
Getting the Most Out of Each Dose
Take Allegra at the same time every day, with plain water, on an empty stomach or with a light meal. Avoid fruit juice near your dose. If you’re using the 180 mg once-daily tablet and find that symptoms return by evening, switching to the 60 mg twice-daily regimen can provide smoother, more even coverage across the full day. Both approaches deliver the same total amount of the drug over 24 hours, just distributed differently.