Just don't over do it. I got used to 2 vivarins at a time. I worked grave yard shift at a drywall manufacturing plant. I decided to take "4" vivarins at once. I thought I was going to die for about an hour. No bullshit, just don't go overboard with the pills. If you get used to a certain amount then quit using it for awhile or if you have to use them, break off part of another pill to add a quarter or half pill.
Another great alternative is to try to sleep more. I find I naturally wake up after about 10 hours of sleep, but then again, it takes that much to recharge me cuz I wake up about once an hour during the night.
I decided to use it to my advantage though. Now I drink a protein shake half way through the night, or about the 4th or 5th time I wake up lol. I swear the military did this too me. Or maybe it's just my pea sized bladder lol, who knows? HAHA.