What Should I Tell My Healthcare Provider Before Taking Diphenhydramine?
You should talk with your healthcare provider prior to taking this medication if you have:
- Stomach ulcers
- Gastroesophageal reflux disease (GERD)
- An enlarged prostate (also known as benign prostatic hyperplasia, or BPH)
- Difficulty passing urine
- An overactive thyroid (hyperthyroidism)
- Heart disease
- High blood pressure (hypertension)
- Chronic obstructive pulmonary disease (COPD)
- Any allergies, including allergies to food, dyes, or preservatives.
Also, let your healthcare provider know if you are:
- Pregnant or thinking of becoming pregnant (see Diphenhydramine and Pregnancy)
- Breastfeeding (see Diphenhydramine and Breastfeeding).
Make sure to tell your healthcare provider about any other medications you are taking, including prescription and non-prescription medicines, vitamins, and herbal supplements.
(Click Precautions and Warnings With Diphenhydramine to learn more, including information on who should not take the drug.)
How Does Diphenhydramine Work?
Diphenhydramine is part of a class of drugs called antihistamines. Specifically, it is an H1 receptor antagonist. This means that it blocks a specific type of histamine receptor in the body (known as H1 receptors). Since allergic reactions are partly caused by the release of histamine from certain cells in the body, diphenhydramine can help relieve or prevent allergy symptoms.
Diphenhydramine also blocks acetylcholine receptors, an action which produces some of the bothersome side effects of the medication (such as dry mouth or difficulty urinating) but also makes the drug useful for some uses (such as for Parkinson's disease or for relieving a runny nose due to the common cold).
Like many antihistamines, diphenhydramine also works as a sedative. This action has made the medication useful as a sleep aid but also limits the usefulness of the medication for other uses (since drowsiness can be a bothersome side effect).