What is the difference between first and second-generation antihistamines?

Side effects. The second generation antihistamines were developed to try and decrease the side effects of the first generations. Many are "purified" first generation antihistamines so they have the active effects without the side effects of drowsiness, sedation, dry mouth etc. They also tend to have longer half lives, meaning they can be taken once or twice a day not every 4 hours.
Sleepyness. The first generation antihistamines (benadryl, chlor-trimeton, tavist) are short acting and usually make people sleepy. Second generation (claritin, zyrtec, allegra, xyzal, (levocetirizine) clarinex) are long acting once a day, and generally do not make people sleepy, or less so.
Antihistamines. 1st generation are better antihistamines but have significant drowsiness. 2nd generation do not bind the receptor as well as 1st generation, but they cause much less drowsiness.

Related Questions

Doctors, what is the difference between first and second generation antihistamines?

Sedation & tolerance. most first generation antihistamines cause sedation and induce tolerance, meaning that if you keep taking them, then you would need higher doses after sometime to have the same clinical effect, that's not the case with most second or third generation antihistamines. Read more...
CNS. The 1st generation AH gets into the brain area thereby blocking the stimulatory effect on alertness from histamine. The 2nd generations ones do not get into the brain and thus do not usually induce drowsiness. 1st gen- more drying and better for runny nose , the sedating effect also better for itching than 2nd gen. Read more...