Viagra is just like any other drug - I don't think it's addictive, but I dont expect it will do any harm, except that you may need to rely on it more and more to get a good erection (an unexpectedly good firm erection anyway)
I wouldn't use it if I were you - if you don't need it, there is no reason to take it especially if you are only 23! I think your new gf would be kind of disappointed if she found out you had to take viagra to have good sex - there is a lot more to sex than the size and firmness of your erection! And any woman worth having sex with understands about performance anxiety and what that is all about! :-)