Dairy allergies are so common these days and a pizza is one great gooey mess of cheese that will aggravate that allergy. Some of the pizza chains are actually using hydrogenated vegetable oils as a replacement for genuine cheese, which I regard as an offence to their customers. SO - check to see if you have a dairy product allergy - and always be sure you are being offered GENUINE cheese in your pizza. If both those factors are O.K., then an occasional pizza won't do you any harm.
It depends on the kind of pizza you eat. Having a slice or 2 of regular cheese pizza is fine...and even better for you if it is thin crust. If you have tons of meat and other stuff then no it is not. It is also not good if there is a lot of grease on the pizza. You could also make sure to not get DOUBLE cheese and other stuff. But don't take it out of your diet....having pizza is fine....just not everyday!! :)