Your vet is being very responsible. As someone mentioned antibiotics don't work on viruses. They are only presribed in those cases for a "possible" secondary bacterial infection. L-lysine is the appropriate prescription for a cat with a "cold" although you can purchase it from a health food store as well. Doesn't have to come from your vet.
Your vet would be trying to make $ off you if he/she was prescribing antibiotics for a cat that had symptoms of a virus.
And as per what sugarcatmom said, L-lysine is not a homeopathic remedy. It is an amino acid. "As an essential amino acid, lysine is not synthesized in animals, hence it must be ingested as lysine or lysine-containing proteins."
Some animals need to stay on while others seem to do well on a course of it. When I adopted my cat Mochie (now 14) from the SPCA she had the typical upper respiratory illness. So often you are told once they have it there will always be recurrances. Nope. I treated all my cats with the L-lysine as well as other natural aids and she's never had a relapse. Not even when I adopted my 2 most recent cats (1 year ago) who came to me with the illness. My other 2 plus the kittens were sick but Mochie wasn't. Once again I treated them all with the amino acid (as per the vets instructions) and no one has had any problems.
Dracko (GSD) male - 2000 - 31/02/2011 RIP my boy
Mochie (Balinese X/feline) female - 1994 - 07/01/2010 RIP my sweetie
Ginkgo (Siamese X/feline) female - 6 years
Smitty (long-haired white/feline) male - 5 years
Teiya (Ragdoll x/feline) female - 3 years
Kinsey (Ragdoll x/feline) female - 3 years
DONíT BREED OR BUY WHILE SHELTER PETS DIE!