Benadryl is a brand name for the drug diphenhydramine [1]. It is an antihistamine that is used to relieve symptoms of allergies, such as itching, sneezing, and runny nose. Benadryl is also used to treat motion sickness, insomnia, and mild cases of Parkinson's disease [2]. However, like all medications, Benadryl can cause side effects.

The duration of Benadryl side effects can vary depending on the individual and the dose taken. The most common side effects of Benadryl include drowsiness, dry mouth, dizziness, headache, and stomach upset. These side effects usually last for a few hours after taking the medication [3]. However, in some cases, the side effects can last for up to 24 hours or longer, especially if a higher dose is taken.

Other less common but more severe side effects of Benadryl include difficulty breathing, hives, and swelling of the face, lips, tongue, or throat. These side effects require immediate medical attention and can be life-threatening.

It is important to follow the recommended dosage instructions and to talk to a doctor before taking Benadryl, especially if you have any underlying medical conditions or are taking other medications.

In conclusion, the duration of Benadryl side effects can vary depending on the individual and the dose taken. The most common side effects usually last for a few hours, but in some cases, they can last for up to 24 hours or longer. It is important to be aware of the potential side effects of Benadryl and to talk to a doctor before taking the medication [1][2][3].

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/BENADRYL
[2] https://www.nhs.uk/medicines/diphenhydramine/
[3] https://www.nationwidechildrens.org/family-resources-education/700childrens/2020/04/benadryl