Yes, our son (9 years old) is currently on Zonisamide -- he's been on it since October, and it works very well in controlling his seizures (he was having several tonic-clonic/grand mal and tonic seizures a day). He'd gone through 9 or 10 meds previously, none of which worked for him.
The most likely side effects you can expect from Zonisamide is loss of appetite, acidosis, and kidney stones.
What we do for the appetite problem (it tends to cause tummy upset too) is give him his full dose for the whole day at the end of his supper meal. His doc said you can do this with Zonisamide because it stays in the body for a long time. This also works well in helping him sleep well at night (as this med tends to make one sleepy).
To prevent acidosis (symptoms of this will be feeling very tired, and panting) and also to prevent kidney stones, make sure your child is getting lots of fluids. At least 4 cups a day. I add a little lemon or lime to Jon's water (and 1/8 tsp baking soda once a day), and sweeten it with Stevia, as all of these help reduce acid levels in the body.
Your doctor should have done a baseline electrolyte test (sodium, potassium, chloride, bicarbonate) when your child started this med. Then, really, your child should have electrolytes tested every 2 weeks for the first several months. You want to make sure the bicarb levels don't drop below 20. If this happens, it can be quite dangerous for your child, and can cause bone loss and lots of other serious health problems. We did have a problem with Jon's bicarb levels for several months (they dropped down to 16), and have to work very hard with having him drink lots of his "lemonade" and take the potassium cytrate to keep it at a healthy level.
Many doctors will prescribe potassium cytrate to children on this Zonisamide to help prevent acidosis and kidney stones. Cytra-K is one that is used. Currently, Jon just takes potassium cytrate tabs.
Other side effects we've seen is some cognitive dulling and some aggression. Jon is on a very high dose (a very low dose controlled the grand mal seizures, but we had to take it up high to control the tonics) -- we didn't see the cognitive dulling at a lower dose. Now that he has been seizure free for 5 months, we're going to discuss with his doctor maybe reducing the dose slightly to alleviate some of the negative side effects.
