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

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.