Hi
Guinea pigs don't get viral cold; they get a bacterial respiratory infection (URI), which needs to be treated with an antibiotic (but NEVER a penicillin based one); enrofloxacin, trimethoprim-sulfamethodazole or doxycyline based antibiotics are safe to give. An un- or undertreated URI can cause permanent damage or kill. Multivitamins won't cure a URI.
Here is our very practical syringe feeding and medicating guide with all the little tricks and tips:
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Here is more information on URI:
Guinea Lynx :: URI
Safe and unsafe medications for guinea pigs (if necessary to show your vet):
Safe medications including recommended dosage calculation for your vet):
Guinea Lynx :: Medications
Unsafe medications for guinea pigs:
Guinea Lynx :: Dangerous Medications
You may also find our new owners information collection very helpful:
Getting Started - Essential Information for New Owners
All the best! Please do not underestimate a respiratory infection in guinea pigs; it is NOT just a little cold!