Do you have to have a bass amp for a Bass guitar, or can you use a guitar amp?

You can use a guitar amp with a bass guitar but you could/would have 3 problems I can think of immediately:

1. Your sound won't be a rich as it would be using a bass amp. Bass amps are made for that low end that a regular guitar amp can't attain.

2. Your sound could likely be distorted, which might be ok if you're going for hard rock, but if you want a pure sound, it wouldn't be good.

3. You could blow out the guitar amp altogether. The guitar amp might not be able to handle the sheer force of your bass and your bass playing and it could cause a blowout.

Without knowing specifically what kind of guitar amp you're using, these are the issues you could run into. To answer your question - yes, but you have to be careful.

Technically you can use a guitar amp if you have to, but eventually you will blow it out. It just isn't made to handle that frequency. Plus it just isn't going to sound right.