LdyJane put it perfectly. I first took Zonegran when I decided to stop taking topamax because the side effects were just about as unbearable as the headaches. My neurologist (at the time) pitched it as a drug that's very similar to topamax but typically has milder side effects. I found that to be right on. Others feel very differently, as you'd expect with any of these preventatives.
In my experience with zonegran (in taking it for something like 3-4 months the first time around and more than 6 the second), the drug seemed to prevent attacks about as well as topamax did. At times I actually experienced some very good results with both of these meds. However, I discontinued Topamax after only 3 or 4 months because, as my headaches got worse--as they continued to get for several years to present date, I had to step up my dose to a very high level. At the highest that I went my headaches significantly dulled in intensity (from maybe an 8 to a 5 or 6) and I mostly was able to stay at that baseline and have very few spikes throughout the day. However, I was practically mindless and I literally wasn't eating. And when we dropped the dose to a bearable level, I resumed having an 8 or so around the clock.
Now, to contrast that with zonegran. One important point is that I did stop taking both of them because of side effects and not enough relief. But things went very differently with Zonegran. The first time around I only went up to a moderately high (speaking very relatively) dose. I stayed at that dose for a while and achieved some of the same relief that topamax brought me. But after a few months I decided it wasn't helping enough and (being at the highest recommended dosage) moved onto something else. During that time the side effects were very mild compared before. I was still somewhat dullwitted and I experienced a bit of parathesia (tingling/pins-and-needles). But, without sufficient headache prevention I couldn't stay with it.
The second and longer time I was put on it again after having a bad experience with topamax (new neurologist). But that time I went up to a MUCH higer dose. It worked to some degree, though I stayed on it for so long mainly because I had exhausted nearly all other drug treatment options. At that really high dose I felt, as best as I can describe it, "half there", or less. I couldn't think straight, my balance was way off, etc.
[I've rambled a whole lot, sorry--here's the main idea]
The important point is that, for me, zonegran was indeed very similar--in its preventative capacity--to Topamax. At high doses both drugs were very cognitively impairing and had other not-so-fun side effects. BUT, zonegran seemed to make getting through the days more difficult, in a quicksand-ie way making me have to push hard to do complex tasks (and some not so complex ones). Topamax didn't just slow me down--it made me fully stop. Not slow thinking, nearly no thinking; not just irritable but emotionally debilitated. So, if zonegran works for your headaches it's much more bearable, as most people will tell you.
Just keep in mind that my experiences with both of these medications happened while I was at VERY high dosages. I was monitored very closely by several neurologists at the Jefferson Headache Center because I was taking significantly more than the "maximum daily dose" in those prescription information pamphlets. And I'm really describing how things were largely at the most extreme cross sections of the time I was taking each medication, with the intent to contrast the differences between these cousins at their "most potent". So, remember, most people do just fine and won't experience the worst side effects. Unless you get put on a radical dosing regime you can probably expect some of the side effects that I described at much lesser magnitudes....I'm afraid that I made things sound really intimidating, so just remember that I mention the worst just to show the difference. In my opinion zonegran is a fantastic option, even though it didn't end up working out for me.
DX: NDPH, Recovered CRPS
RX: Lamictal, Provigil, Clonazepam, Ambien CR, Emsam, Namenda, Oxycontin, Oxycodone
PRN: Haloperidol, Zyprexa, Lodine, Zofran, Skelaxin