They are the most used class of medication in the USA and the 07th in the UK.
The first one discovered in 1937 was a compound called 933F, followed shortly after by 929F.
The big problem is that both were very toxic to the human being.
In 1942 Antergan and in 1945 Diphynhidramine were discovered.
Second generation were only developed in the 1980s.
Currently, there are over 40 different ones.
To make matters slightly more confusing, some second generation also have first generation properties.
But we are only talking about the ones acting on H1 receptors.
These are used for all sorts of allergic reactions:
What are the main differences?
In reality, there are 4 different types of histamine receptors (from H1 to H4).
We have antagonists (actual name is reverse agonists) for H1 and H2, but not for the others.
H1 are further divided into 7 different chemical groups.