Active Pharmaceutical Ingredients

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Diphenhydramine Hydrochloride

147 – 24 – 0

\(C_{17}H_{21}NO\cdot HCl\)

291.82

The parent compound diphenhydramine has a structure with two phenyl rings attached to a central ethylamine group. In the hydrochloride salt form, the nitrogen atom of the ethylamine group is protonated and forms an ionic bond with the chloride ion. The chemical structure is \(C_6H_5CH_2CH_2N(CH_3)(C_6H_5)\cdot HCl\).

Appears as white to off – white crystalline powder.

Highly soluble in water, soluble in alcohol, and slightly soluble in chloroform. Its good solubility in water makes it suitable for various pharmaceutical formulations such as tablets, capsules, and liquid suspensions.

First – generation antihistamine. It also has anticholinergic and sedative properties.

Diphenhydramine works by competitively blocking the H₁ histamine receptors. By binding to these receptors, it prevents histamine from exerting its effects, such as vasodilation, increased vascular permeability, and smooth muscle contraction. The anticholinergic effects are due to its ability to block muscarinic acetylcholine receptors, which contributes to some of its side effects like dry mouth and blurred vision. The sedative effect may be related to its action on the central nervous system, possibly by inhibiting the release of neurotransmitters involved in wakefulness.

Allergic Reactions: Widely used to treat allergic conditions such as hay fever, urticaria (hives), and allergic rhinitis. It can relieve symptoms like itching, sneezing, runny nose, and skin rashes. Sleep Aid: Due to its sedative properties, it is sometimes used as an over – the – counter sleep aid. Motion Sickness: Can be effective in preventing and treating symptoms of motion sickness, including nausea and vomiting.

Stable under normal storage conditions, but should be stored in a cool, dry place away from light. Exposure to moisture, heat, or excessive light may cause degradation over time.

Common side effects include drowsiness, dry mouth, blurred vision, dizziness, and constipation. These side effects are more likely to occur, especially in higher doses. In some individuals, it may cause paradoxical excitement, especially in children. Prolonged or excessive use can lead to more serious complications, and it may interact with other medications, such as antidepressants and antipsychotics, increasing the risk of adverse effects. It is also important to note that driving or operating heavy machinery should be avoided while taking diphenhydramine due to its sedative effect.