When it comes to dosing guidelines and how long Benadryl takes to start working, the answer is relatively simple. This popular sleep aid and allergies relief drug typically starts to exert its effects within 20 minutes of taking it – making it a go-to for those suffering from sniffles or insomnia. But its uses don't end there; this over-the-counter medication can also be used as an antihistamine to reduce swelling and itching caused by hives, rashes, bug bites, and other minor skin irritations. As with all medications though, you should always check with your doctor first before using since Benadryl may interact with certain drugs.