It is extremely dangerous to give any medication to a cat without consulting with a veterinarian. Cats are highly sensitive to many medications that are safe for humans, and penicillin can be toxic to them.
Never give your cat any medication without first getting the approval and specific dosage instructions from a qualified veterinarian.
Please contact your veterinarian immediately if you have any concerns about your cat's health or medication. They can provide the best advice and treatment for your pet.