I know the caffeine in diet pepsi can affect weight loss and gain. However, is there another ingredient that is harmful in diet pepsi to my health?

It has been shown, through numerous scientific studies, that ANY artificial sweetener can have negative long term effects on the health of humans. Most have been linked to cancer! Try this: Find what sweetener is used in whatever product you are interested in, then use Google (the Internet's largest search engine), and put both the sweetener's name AND the word cancer on the same line in the search box, like this:

Aspertame cancer

Then browse through the results! By the way, the effect of ALL diet drinks on weight gain, is that your taste buds can be fooled, but your digestive system cannot be fooled, so once it discovers that there is NO sugar in your drink, it will trigger your hunger system to EAT and DRINK even more, until your appetite is satisfied! The result is you WILL overeat! Blood sugar level is the key.