Dr. Shelly Rubin—who treats Oprah's dogs—says spaying or neutering is one of the best things you can do for your pet's health.

For males, Dr. Rubin says neutering can change the dog's behavior for the better. "There's no question that if you neuter a male dog, you're going to do several things. One, you'll reduce prostate cancer and testicular cancer," he says. "Most important, you'll make that dog a really good house pet because it won't be chasing after females in the neighborhood. It listens to you when you speak to it, and it won't be marking urine all over the neighborhood—and your house—in many cases."

For females, Dr. Rubin says it's best to spay them before first heat. "If you spay them before their first heat, and this is scientifically proven, you can reduce the incidence of breast cancer when they're older by somewhere near 90 percent," he says. "So why wouldn't we want to spay and neuter our pets?"

Dr. Rubin says spaying and neutering are very simple procedures. After a quick exam to make sure the animal is healthy enough for surgery, it's given a sedative and anesthesia. The procedure take about 10 to 15 minutes, and the animal can go home that day. "While it costs some money to do it on a private level, there are many, many free or low-cost spay and neuter clinics all over the United States," he says. "That cannot be an excuse for not spaying or neutering your pet."