A 50-milligram dose of diphenhydramine is safe for an adult, as Drugs.com reports. For cold and allergy symptoms, an individual can take this dose every four to six hours, but he should not exceed 300 milligrams within a 24-hour period.
To avoid overdose, individuals must use caution when combining diphenhydramine with two or more products containing the same active ingredient. For example, pain relievers, fever reducers or decongestants often contain diphenhydramine, according to MedlinePlus.
Common diphenhydramine side effects include dry mouth, dizziness, vomiting, constipation and headache. Serious side effects include problems with vision or trouble with urination, as advised by MedlinePlus.