Hi Siti,
Inappetence and lethargy are serious medical signs that need actual veterinary medical check. This is to have an accurate diagnosis or identification of the actual medical situation of your cat to have a sound and effective intervention.
There are supplements that may help boost the immune system as well as help increase the appetite of your cat, but these are not in lieu of an accurate veterinary medical check.
For other related information, please visit and talk to ou Pet Care Consultants and for veterinary medical intevention, refer promptly to your veterinarian of choice.
Best regards,
Eliezer M. Bauzon, DVM RN