What Are The Side Effects Of Zentel?

My 3yr old boy (almost 4) has been prescribed Zentel. To take 20ml tonight; only one dose. I need to know how fast will I see results, would he have diarrea? Should I keep him from school tomorrow. The dr was very general about what to do as to say thatthere is nothig to woory about, other than watch for worms... Please, help1