Final answer:
Antihistamines are effective in reducing allergy symptoms by blocking histamine receptors, but they can also affect other receptors causing side effects like drowsiness, dry mouth, and blurred vision. First-generation antihistamines are more likely to cross the blood-brain barrier and have anticholinergic effects.
Step-by-step explanation:
Antihistamines are designed to combat allergy symptoms by interfering with histamine action in the body. However, because some antihistamines are not completely selective for just one type of histamine receptor, they can also affect other receptors, leading to additional symptoms. Antihistamines primarily block the action of histamines at H1 receptors, which can alleviate symptoms like itching, runny nose, and sneezing associated with allergic reactions. While this is beneficial in reducing unwanted allergic responses, it can also cause drowsiness, dry mouth, and blurred vision as side effects because these medications may cross the blood-brain barrier and also inhibit acetylcholine, a neurotransmitter. Histamine H2-receptor antihistamines target receptors in the stomach to reduce acid secretion and are used in the treatment of acid reflux and ulcers.
The chemical structure of some antihistamines allows them cross the blood-brain barrier, leading to effects such as sedation. In addition, some first-generation antihistamines have anticholinergic properties that can cause symptoms like dry mouth, urinary retention, and constipation. Newer second-generation antihistamines are designed to be more selective for the H1 receptor and do not cross the blood-brain barrier as readily, minimizing central nervous system side effects and making them a preferable choice for many individuals seeking allergy relief.