what kind of tablets was Samuel L. Jackson taking?

At 50min 30 seconds into the movie, Jacksom offers her a pill from his bottle, "Like one of these? Keep you going? No?"
He then stuffs a few loose tabs in her upper jacket pocket. "Just in case. For later."

I think we can definitely say they were uppers. likely Provigil (Google it). A non addictive, non jittery upper that even some high level USAF pilots use like the B1 bomber pilots who fly 20 hour plus missions with many in fight refueling. They are never based on foreign soil or do they land there. They are deployed only from the USA and fly these long hour and mileage missions with two pilots. One will often sleep on an inflatable air mattress when possible.

The Flight Surgeon prescribes Provigil for these missions.

Provigil is a hot illegal seller on aircraft carriers among fighter jocks who often NEED them but are not prescribed them because of stupid PC reasons. There is no drug screening for Provigil so the Military looks the other way as long as no one is caught red handed.

Provigil is light years ahead of amphetamines. Not addictive, no real bad crash, super high concentration level. Actually called a "smart drug". Used by University, law and med students.