From the Research
Benadryl (diphenhydramine) is not typically associated with causing bradycardia (abnormally slow heart rate), and its use is more commonly linked with tachycardia due to its anticholinergic effects. The medication works by blocking histamine H1 receptors and has significant anticholinergic properties that tend to increase heart rate rather than decrease it. At normal therapeutic doses (25-50mg every 4-6 hours, not exceeding 300mg daily for adults), cardiovascular effects are usually mild 1. However, in overdose situations or in certain sensitive individuals, particularly elderly patients or those with pre-existing cardiac conditions, paradoxical effects might occur.
According to a study published in 2025, diphenhydramine has a higher adverse side-effect profile, especially among children and older adults, which has led some countries to restrict access to first-generation antihistamines like diphenhydramine 2. Another study from 1995 found that diphenhydramine produced bradycardia followed by a transient hypertensive phase without affecting the QTc interval in guinea-pigs, but this effect is not typically seen in humans at therapeutic doses 3.
If you experience unusual heart rate changes while taking Benadryl, you should contact your healthcare provider. This medication can also cause drowsiness, dry mouth, blurred vision, and urinary retention due to its anticholinergic effects, which may be more concerning than any direct cardiac effects for most users. Given the availability of second-generation antihistamines with similar efficacy and fewer adverse effects, the use of diphenhydramine should be approached with caution, especially in vulnerable populations 2.