For my four, lettuce is a primary food in their diet. They enjoy:
Romaine
Little Gem
Radicchio
Frisee
Endive
Escarole
MIzuna
Lamb's
Rocket
Lollo rosso
Red batavia
And more. They love the lettuce mixes although it isn't quite the same as having a big leaf to rip through.
Mine have had loads of lettuce each day (minimum 1 full romaine leaf per pig - more commonly half a lettuce mix plus 2-6 leaves per pig). No ill effects at all. Poops are the best they have ever been (!) - no diarrhea for over a year - and they have only been to the vets twice in the past year.
And as far as the "sedative" thing goes (laudanum), my pigs are completely unaffected by the "excessive" amounts of lettuce I feed. As energetic and content as ever.