The most common gauges of needles for cats and small dogs in order of popularity are: 20 gauge, 21 gauge, 19 gauge, 22 gauge. Most vets use the same Monoject 18 gauge for 200 pound dogs and 5 pounds cats. Using the same large needles may be fine for veterinary clinics because they often give fluids in emergency situations and typically only 1 time. When giving fluids on a regular basis it is important to use the optimal needle gauge. ThrivingPets does not recommend 18 gauge needles for pets under 50 pounds. ThrivingPets stocks Terumo needles, McKesson / Nipro needles, and Nipro Needles. Please contact ThrivingPets if you have any questions on needles, IV sets, or fluids online, email- email@example.com, or phone 303-333-1899.