Final answer:
1st generation H1 antihistamines often cause drowsiness and affect both the brain and the rest of the body, while 2nd generation H1 antihistamines are less likely to cause sedation and are more selective in targeting receptors outside the brain.
Step-by-step explanation:
1st gen and 2nd gen H1 antihistamines refer to two generations of medications that block the effects of histamine, a chemical involved in allergic reactions. 1st generation antihistamines, such as diphenhydramine, are known to cross the blood-brain barrier, leading to drowsiness, and they bind non-selectively to H1 receptors in both the central nervous system and the peripheral nervous system. In contrast, 2nd generation antihistamines, such as cetirizine, are much more selective for peripheral H1 receptors, reducing the chances of drowsiness and are less likely to cross the blood-brain barrier. Both are used for the treatment of allergies, while 1st generation can also be utilized for insomnia or motion sickness, and 2nd generation antihistamines are typically preferred due to a lower incidence of sedation.