My son was on Prevacid from 2-6 months. I would totally use it again. I didn't see any side effects, except that around 6 months he started spitting up more! We upped his dose and it got worse. Finally, one day, I just didn't give it to him, and it was MUCH better. Weird, huh? He did still spit up a little (until he was almost a year old), but not enough to require medicine. I guess his body just had enough of it and was giving up the reflux itself. So, just watch your baby, follow what the doctor says, but ultimately try what you think might work for your baby.
Again, I know he needed it while he was on it, and it helped so much until the very end, so I would do it again in a heartbeat!
Also, many people will talk to you about the milk allergy or that it may be something you are eating and getting into your breastmilk, and it truly could be, but it wasn't for me. I changed many things in my diet including taking out all dairy, and nothing helped. And...he definitely does not have a milk allergy. So, try all the changes for a little while, if you think it might help, but don't worry if not. For most babies, including mine, he kept down breastmilk much better than formula, no matter what I ate. The prevacid should really help.
Hope it does!!
EDIT - I read another post and remembered...my pedi told me that prevacid only reduces the acid that makes the reflux painful...it doesn't necessarily reduce the amount of spit up. We chose to go on the prevacid because my son was in much pain. On the prevacid, I think he did spit up less, but he still did spit up alot (compared to other babies around him), he just didn't care. Once I took him off, he was spitting up less and it still didn't hurt him, so that's why I kept him off after 6 months. You're still stuck with a life of laundry and burp rags in every inch of your house even on the medicine...sorry!