The proper dosage of penicillin (or amoxicillin) for a cat depends on several factors, including the cat's weight and cause for infection or illness, according to Drugs.com. For example, cats with renal dysfunction may need a smaller dose than what is recommended for their weight, while cats with severe urinary or gastrointestinal tract issues may need a higher, more concentrated dose. A veterinarian can examine the cat and its condition to determine a proper dosage.
A prescribed penicillin or amoxicillin treatment should be given every day for a period of 5 to 7 days. This schedule should be followed even if symptoms are no longer evident. In general, the typical dosage of amoxicillin is 11 to 22 milligrams per 2.2 pounds of the cat's weight each day. If basing the dosage on this scale, the following should be considered for determining the proper amount (an amoxicillin tablet is typically 100 milligrams).
For cats weighing under 4.96 pounds, give 1/4 tablet. For cats weighing 4.96 pounds, give 1/2 tablet. For cats weighing 9.9 pounds, give 1/2 to 1 tablet. For cats weighing over 9.9 pounds, give 1 tablet.
Infections that require higher doses, such as a urinary tract infection or gastrointestinal tract issue, the veterinarian may double the dose for the cat.