I moved your thread to a more appropriate section.

Snuffles isn’t really a disease, it is a symptom. Did your vet do any kind of testing? Or did they just prescribe the PenG shots? Penicillin is generally not the “go-to” for URIs. Most vets seem to start with Baytril or another antibiotic that can be given orally (penicillin CANNOT be given orally). When it comes to rabbits though, it seems they often need longer courses of antibiotics that what would “normally” be prescribed. This can be due to a variety of reasons, but if he got better and then his symptoms returned then that tells you that the course of treatment was not long enough. If it were me, I would inquire with the vet about continuing antibiotic treatment. You may not need a real appointment for this.

As for the Echinacea, it won’t hurt… but there is limited evidence that it will help. I don’t think it could cure an active URI, but it might help keep a bunny that is prone to URIs from getting them so often. If you do give it, I would suggest giving a supplement that is meant for rabbits. Small Pet Select makes one and they provide a feeding suggestion on the package.