Be cautious when taking Benadryl — the side effects could be serious.

While Benadryl might seem like a fairly safe drug — and for the most part it is — taking too much of it can cause serious problems. Antihistamines like Benadryl work by drying up mucous membranes that help you with the sniffles, but those membranes are also vital for a healthy immune system. If your cold has you taking more Benadryl than is recommended, see your doctor.