Will an 8-jin cat be poisoned by 0.5 g of human amoxicillin?

Cat amoxicillin is usually safe if dosed right, but human doses are hard to control and carry poisoning risk, worse if kidney or liver function is poor. The pet dose is 12.5 to 25 mg per kg, so an 8-jin (4 kg) cat needs about 50 to 100 mg.

You gave 0.5 g of human amoxicillin, which is excessive, though it depends on that product’s strength; watch the cat closely. Toxicity shows as lethargy, no appetite, vomiting, drooling, breathing trouble, even seizures or coma, and can be fatal.