We have looked at side effects of feed additives (FEED ADDITIVES: DO THEY ADD TO ANIMAL WELFARE? AN EVALUATION . C.A. Kan, L.P. Jager and J.F. Grommers Animal Welfare I998, 7: 397 -414)

I am not aware of similar papers on "real" drugs.

Any effect wil always depend on the kind of trial you are doing and what parameters you are measuring

Article Feed Additives: Do They Add to Animal Welfare? An Evaluation

We have looked at the effects of short-term loss of feed access in broiler chickens on gut flora in an attempt to relate this to long-term susceptibility to enteric disease. This was done using standard and antibiotic-free feeds in modern broilers and in a random-bred strain representative of 1978 broiler genetics. Anitibiotics were included at the prophylactic level (as a growth promotant, level is a very small fraction of what would be provided therapeutically). Here is an exerpt from the project final report:

"Key points to take away from this project are that the changes to the genetics of the modern broiler are the culmination of changes across the board within the biology of the bird. Growth promoting antibiotics DO promote growth, even under pristine rearing conditions. This needs to be considered in antibiotic replacement programs. Basically, the modern broiler is more susceptible to disease than in the strain representing 35 year old genetics. There were no differences in Salmonella or Clostridia strains in the gut sections tested. The modern birds were therefore not more ‘challenged’, and this was also not affected by a 24 hour feed withdrawal. The differences in the ‘good’ bacteria may be the key. Lactobacilae were affected by presence of antibiotic, and Bifidobacteria were affected by strain and feeding. The fact that the strain effect was limited to the ‘good’ bacteria and not the ‘bad’ bacteria measured provides evidence for an altered susceptibility to gut infection in modern, growth-selected broiler strains. This suggests that research into probiotics or prebiotics supporting growth of the beneficial gut flora may be a key area of research as antibiotic bans draw closer." 

The antibiotic growth promoter we used in the above work was, Bacitracin Methylene Disalicylate (BMD).

Antibiotics as growth promoters are out . . .

Antibiotics as a treatment might be essential sometimes.

Of, course AB will be used in TREATMENT of infectious diseases, but here it was AB used as a growth promoter and "prophylactic level". That is now banned in EU, and has newer been used in Norway. Perhaps strange, because a norwegian company produced zinkbacitracin 50 years ago, but it was never used in our country. If the livestock production worldwide continue to use AB in this manner and in this amount, sooner or later the human health authorities will stop this production . . . Look at the amount of AB used/kg meat produced  in different countries . . . So, STOP before it is too late . . .