Catnip is an ancient herb, known to humans for at least 2000 years. Its effect on cats is generally pleasant, making the cat more playful. This effect is non-addictive and temporary and lasts only about 15 minutes after which it simply stops affecting the cat.
Cats usually appear relaxed and have a nap after playing with catnip.
Because most cats are attracted to it, catnip is useful in attracting cats to their toys, scratching posts, etc. If you move to a higher-end diet you will usually have no problem getting your pet to eat since these foods usually have very high palatability. The key here is usually to avoid overfeeding your pet. When facing a highly palatable pet food most pets respond by eating a more than is good for them. Check your dog's stools to make sure that they are firm and not loose.