Doxycycline as a “morning after pill” will not prevent pregnancy (although it sure can reduce the effectiveness of birth control pills!) but it can prevent or treat sexually transmitted diseases. Some women actually take antibiotics after sex, to reduce their chance of getting UTIs from this if lesser measures don’t work.
A major issue in antibiotic resistance is their misuse in livestock production. Got a sick animal? Medicate them, you bet, but giving poultry a homeopathic dose of tetracycline so it will reach butcher weight a day or two early (this does not, to my knowledge, increase growth in other animals) should not be done routinely.
ETA: Ivermectin does not work for cryptosporidiosis, the water-born diarrheal disease that’s getting a lot of press right now.