who do you tell when a prescription drug causes damage?

[Ree]

my husband was prescribed an anti biotic we were not warned of any side effects but he became so unwell mentally that I went onto the web and found loads of dedicated sites all saying about damage this drug can cause even after 1 dose. We are unable to get an answer as to how long he is likely to be ill all we can get is until the drug leaves his system but it has been 3 weeks already, where can I go to find more info, have we got a drug safety agency or any such organisation that can help