Benadryl (diphenhydramine) typically lasts 4 to 6 hours per dose in most adults. Taking two tablets means you've consumed a double dose, which extends neither the duration nor the intensity proportionally—it simply increases the total amount of the drug in your system at once, creating a stronger effect over roughly the same timeframe. The medication begins working within 15 to 30 minutes, reaching peak levels in 1 to 3 hours, then gradually declining. Several factors affect how long Benadryl remains effective: age (it lasts longer in older adults and children), liver function, body weight, whether you've eaten, and individual sensitivity. Some people metabolize it faster or slower than average. The sedative effects—the drowsiness most people associate with Benadryl—may wear off before the antihistamine effects fully disappear from your body. For ongoing allergy or sleep management, Benadryl is typically taken every 4 to 6 hours as needed, but you should follow package directions or your doctor's guidance rather than self-adjusting doses. If you're considering taking double doses regularly, consult a healthcare provider, as long-term regular use of diphenhydramine carries potential risks including anticholinergic effects and tolerance development.