Yes, it can because the antibiotics kill off helpful flora in the intestines along with whatever bug the vet is trying to treat with the antibiotics. This is why dogs can get upset stomachs and diarrhea from antibiotics. Happens in humans, too. If he starts eating less because his gut feels awful, he’ll lose weight. If the weight loss, nausea and loose stools are being caused by the antibiotic, it should stop within a few days of discontinuing the antibiotic. If your dog is prone to bad reactions to antibiotics, pick up a bottle of ProBios to give alongside the antibiotic. This repopulates the helpful flora in the GI tract so the dog doesn’t have the reaction to the antibiotics. Also call your vet to double-check whether this is a bad reaction to the antibiotic or being caused by the original illness. In both cases, the vet may want to either examine your dog again or prescribe a different antibiotic. Good luck!