Benadryl (diphenhydramine) is one of the most popular over-the-counter medications used to treat allergies, cold and flu symptoms, and various other conditions like insomnia and motion sickness. But a common question people have is how long does Benadryl last after taking it?
Understanding the duration of Benadryl’s effects and how long it stays in your system is important for using this medication safely and effectively.
This comprehensive guide provides detailed information on how long Benadryl lasts, key factors that influence its duration, dosing guidelines, and more.
Key Takeaways on How Long Benadryl Lasts
- The effects of Benadryl typically last 4 to 6 hours when taken as directed.
- Benadryl can remain in your bloodstream for up to 14 hours and fully clear your system in 1-2 days.
- Higher doses lead to more powerful effects that last longer. The maximum effect occurs around 1-2 hours after taking it.
- Age, weight, metabolism, tolerance, and other medications can all impact how long Benadryl stays in your body.
- Benadryl’s sedating effects usually last longer than its antihistamine effects. Drowsiness can persist for 6-8 hours.
- Time-release Benadryl capsules have prolonged effects, lasting up to 12 hours.
How Long Do the Effects of Benadryl Last?
When used as directed, the effects of regular immediate-release Benadryl tablets, capsules, or liquids typically last 4 to 6 hours.
However, the duration of effects can vary quite a bit from person to person. Factors like age, weight, individual metabolism, medication tolerance, and more all impact how long the diphenhydramine in Benadryl remains active in your body.
Here’s an overview of how long Benadryl’s effects usually last:
- Peak effects – 1 to 2 hours after taking a dose
- Antihistamine effects – 4 to 6 hours
- Sedating effects – 6 to 8 hours
- Complete wearing off – 8 to 12 hours
The antihistamine effects that help treat allergy symptoms tend to wear off faster than the sedative anticholinergic effects that cause drowsiness.
So even once the anti-allergy benefits diminish, the drowsiness and impairments of Benadryl can persist for a number of additional hours if you took a higher amount.
How Long Does Benadryl Stay in Your System?
Even once you can no longer directly feel the effects of Benadryl, it takes longer for the medication to fully clear your body.
Diphenhydramine has an elimination half-life of 4 to 6 hours in adults. This means it takes 4-6 hours for your body to eliminate half of the drug.
It takes around 5.5 half-lives for a drug to be completely eliminated from your system.
So based on its pharmacokinetics, Benadryl generally stays in your bloodstream for 14 to 27 hours after the last dose.
The max amount of time Benadryl may be detectable in blood and urine tests is 1 to 2 days.
However, the time for Benadryl to be eliminated can differ based on:
- Age – The elderly clear Benadryl more slowly. The half-life is prolonged to 9-12 hours in adults over 60.
- Individual variation – Metabolism and elimination differ between individuals.
- Frequency of use – More frequent use leads to faster clearance.
- Urine pH – Acidic urine speeds elimination.
- Other medications – Interactions with CYP2D6 inhibitors slow clearance.
So while Benadryl generally stays in your system up to 1-2 days, the specific duration can vary per person.
How Long Do the Effects of Benadryl Last Based on Dose?
The duration of Benadryl’s effects are highly dependent on the dosage taken.
Higher amounts lead to more powerful and longer-lasting effects. However, large doses also increase the risks of dangerous side effects.
Here’s an overview of how long Benadryl’s effects typically last based on dosage:
- 25-50mg – The standard adult dose. Effects last 4-6 hours.
- 50-100mg – Sedation may last up to 8 hours. Do not exceed 100mg in 24 hours.
- 100-200mg – Strong hypnotic and delirium effects for up to 12 hours. Highly dangerous in excess.
- 200mg+ – Potentially life-threatening overdose with effects lasting over 24 hours.
As you can see, taking Benadryl – especially in doses over 100mg – can lead to prolonged drowsiness, cognitive impairment, hallucinations, and delirium-like effects that last for many hours longer than intended.
How Long Does Liquid Benadryl Last?
Liquid forms of Benadryl such as syrups, drops, elixirs, and meltaways all typically contain 12.5mg of diphenhydramine per 5mL.
When taken as directed, the duration of effects is similar to regular immediate-release tablets:
- Children’s Benadryl – Around 4 hours due to the lower 6.25mg per 5mL dose
- Adult liquid Benadryl – 4 to 6 hours
- Meltaway strips – 5 to 7 hours
Liquid and quick-dissolving forms may absorb slightly faster, so the peak concentration in the blood may be reached 30 minutes earlier. But overall, liquid Benadryl lasts just as long as the tablet version.
How Long Does Time-Release Benadryl Last?
Sustained-release and time-release Benadryl capsules exist to provide allergy relief that lasts up to 12 hours per dose.
Some examples of these extended-duration formulations include:
- Benadryl One a Day Allergy
- Benadryl Allergy Ultratab Tablets
- Children’s Benadryl Allergy Dye-Free LiquiGels
These products use special capsules, tablets, and liquigels that slowly release the diphenhydramine over time. This provides longer-lasting effects and allows less frequent dosing.
The duration of effects for time-release Benadryl are:
- Onset of action – 1 hour
- Peak effects – 4 to 6 hours
- Duration of action – Up to 12 hours
So while a regular Benadryl tablet may only provide 4-6 hours of relief, time-release capsules can offer allergy relief that lasts for a full 12 hours.
What Factors Affect How Long Benadryl Lasts?
Many different factors can influence the pharmacokinetics of Benadryl and how long it remains in your system:
- Children clear Benadryl faster than adults. The effects may only last 3-4 hours.
- Elderly people over 60 clear it slower. The half-life is extended to 9-12 hours.
- Larger people process and eliminate it faster due to higher blood volume and liver metabolism.
- Genetics and metabolic differences can significantly alter drug pharmacokinetics between individuals. Some people metabolize Benadryl much faster or slower.
- Liver impairment delays clearance. Benadryl half-life is doubled in cirrhosis patients.
- Kidney impairment has less impact on Benadryl itself, but can increase concentration of its inactive metabolites.
Frequency of Use
- Taking Benadryl often can accelerate clearance and shorten its duration of effects due to enzyme induction.
- CYP2D6 inhibitors like fluoxetine and quinidine impair Benadryl metabolism.
- Anticholinergics like amitriptyline may enhance anticholinergic side effects.
- CNS depressants like alcohol and opioids increase sedation.
- No specific food interactions are known. But eating may delay absorption slightly.
- Acidic urine speeds elimination, while alkaline urine slows it down.
Tips for Maximizing Duration and Effects
Here are some tips to help maximize the duration of Benadryl’s effects when you take it:
- Use time-release formulations – Capsules and tablets that slowly release the dose over 12 hours provide the longest relief.
- Take with food – Eating a meal may slightly extend absorption time and the duration.
- Avoid acidic beverages – Drinking citrus juices and sodas that make urine acidic will reduce half-life.
- Split the dose – Take half a Benadryl dose first, wait 4-6 hours, then take the other half for sustained effects.
- Avoid enzyme inducers – Smoking, phenytoin, and carbamazepine may speed metabolism.
- Manage interactions – Watch out for other anticholinergic drugs and sedatives that enhance effects.
- Lower urine pH – Substances that make urine more alkaline prolong elimination.
- Use caution if elderly – The duration may already be extended in older adults.
FAQ: Answering Common Questions
How long does a Benadryl high last?
The “high” effects from misusing very high recreational doses of Benadryl may last as long as 6 to 12 hours due to its long half-life. Harmful side effects like delirium can persist for up to 24 hours after overdose.
Can I take another Benadryl if the effects wear off?
Yes, you can take another dose after 4-6 hours if needed. But do not exceed the maximum daily limit (200mg per day for adults or 150mg for age 60+).
Does Benadryl last longer on an empty stomach?
No, food does not significantly impact Benadryl’s duration. In fact, taking it with food may slightly extend the effects.
How long after taking Benadryl can I drive?
You should not drive or operate heavy machinery for 4-6 hours after taking Benadryl due to sedation, impaired coordination and cognition. Give yourself adequate time to sober up.
Why does children’s Benadryl wear off faster?
Children’s Benadryl contains a lower 12.5mg dose and kids metabolize it faster, so it lasts around 3-4 hours compared to 4-6 in adults. Their smaller body size also eliminates it quicker.
The Bottom Line
In most cases, you can expect Benadryl to provide 4 to 6 hours of allergy relief and last in your system for up to 14 hours. Time-release forms offer longer 12 hour effects.
Higher doses lead to more powerful sedation that lasts for 6-8 hours but should be avoided due to risks. Benadryl fully clears the body within 1-2 days after stopping.
Pay attention to individual factors like your age, medication interactions, and dose taken that can impact how long Benadryl’s effects persist. Carefully follow dosing guidelines to ensure safe and effective use.