Diphenhydramine precautions. Before using this medication, tell your doctor or pharmacist your medical history, especially of: breathing problems (such as asthma, emphysema), high pressure in the eye (glaucoma), heart problems, high blood pressure, liver disease, seizures, stomach/intestine problems (such as ulcers, blockage), overactive thyroid (hyperthyroidism), difficulty urinating (for example, due to enlarged prostate).