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Both doxylamine succinate and Diphenhydramine belong to the antihistamine group of medicines.

Histamine is a chemical neurotransmitter released in the body in response to conditions like allergies and viral infections. When histamine binds to specific receptors present on the surface of the cells, it causes sneezing, shortness of breath, a runny nose, watery eyes, itchiness, redness of the skin, and swelling of the eyes and lips. When antihistamine drugs like doxylamine succinate and Diphenhydramine are administered, they compete with histamine to bind with the receptors. As histamine is not able to bind with the receptors, it is no longer able to produce the undesirable symptoms associated with it.

Both doxylamine succinate and Diphenhydramine are used to treat conditions like allergic conjunctivitis and allergic rhinitis, hay fever, common cold, urticaria (hives), and itching.

Apart from the antihistaminic effect of doxylamine succinate and Diphenhydramine, both these medicines also exert an anticholinergic effect. This means that they block the action of another neurotransmitter known as acetylcholine. Because of this property, these medicines are also used to treat motion sickness and mild cases of drug-induced Parkinsonism. However, the anticholinergic action can also lead to unpleasant side effects like palpitations, irritability, dryness in the mouth, nose and throat, blurring of vision, urinary retention, thickening of bronchial secretions and constipation.

Doxylamine Succinate and Diphenhydramine as Sleeping Aids

Doxylamine succinate and Diphenhydramine are also known to induce drowsiness and sleep. Therefore, they form a part of many commonly available over-the-counter sleeping aids like Nytol and Tylenol P.M. While other medicinal ingredients -- like benzodiazepines, which are used to aid sleep -- can be habit forming, there is no such problem with doxylamine succinate and Diphenhydramine.

Diphenhydramine has been approved by the FDA for treating insomnia and is frequently used in over-the-counter sleeping pills. This is because makes most people drowsy after a single dose. The extent of drowsiness varies from person to person. Some people fall asleep after taking a single dose, and others require a second dose in order to sleep. Since it is cheap and effective, it is ideal for a person who has never used an over-the-counter sleeping pill before. Diphenhydramine is metabolized quickly and is eliminated rapidly. Therefore, there is no hangover effect the next day. However, tolerance to this medicine develops fast and it may lose its effectiveness as a sleeping aid after a few days of use.

Doxylamine succinate is believed to be more potent than Diphenhydramine, causing marked drowsiness in most people after a single dose. However, its main drawback as an over-the-counter sleeping aid is that it is eliminated from our body at a slower rate. This means you can sleep longer. But it also causes a hangover effect and residual sedation, and you may end up being groggy the next day. Unlike Diphenhydramine, patients do not develop a tolerance to doxylamine succinate. Therefore, it can be used as a sleeping aid for a longer period.

It is important to remember that over-the-counter sleeping aids should not be used for more than two weeks to treat insomnia. If the sleeplessness persists for more than two weeks, you should see your doctor as it may be a sign of an underlying and potentially serious medical problem.

Doxylamine succinate has been classified by the FDA as a Class B medication. This means that it can be used safely even during pregnancy, unlike benzodiazepines which have been associated with birth defects like cleft lip.

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