You can purchase Fishmox (Amoxicillin that is legal to purchase without a script for fish) in their food. You mix 500mg (two 250 mg capsules) with 10ml of water, and give each cat 1ml in their food twice a day for 7-10 days. You can purchase an ml syringe and it's quite easy to measure, mix, and store in the fridge. More @Wikipedia
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