In general these types of anti-seizure medications have a wide-range of side effects typically including lethargy among other stuff. While I don't think that your Levetiracetam (Keppra) is going to "block" the effects of Methamphetamine directly, I would be surprised if it didn't significantly augment the effects subjectively. If you're epileptic, as I'm sure you know, activities like stimulant use is not a good idea. I'm not telling you what to do, but as someone who cares and struggles with seizures myself, I would not recommend it.