Throughout the show's run, Monica rotates through a seemingly never-ending number of restaurant jobs. Despite working as everything from a sous chef to a lunch chef to a head chef at what it's clear are some swanky high-end joints, she never has any money. She has to ask her brother for a loan when she loses her job, she lives with a roommate in a rent-controlled apartment, and despite having spent her entire life planning her wedding, she doesn't have any money saved. Coupled with the fact that she's not a big spender or party animal, the most likely explanations we're left with are that either that she can't stop splashing her cash on nose candy or that Central Perk charges $50 for a latte mocha.
Plus she works in the restaurant industry, which as we all know from tearfully watching reruns of Parts Unknown is fraught with substance abuse ... although you'd need something to mellow you out too if you had to take a fine steak and grill it well-done for any philistine who walks through the door.
Joey, meanwhile, is easier to diagnose. He's not really dumb, just constantly stoned.
Joey is basically your college roommate, a pothead who can't stop eating your food when he's not falling asleep or missing the point of every word spoken at him. He also, like Monica, works in an industry rife with drug usage (the entertainment industry, those liberal heathens). Plus, it's interesting to note that he gets dumber and dumber as the seasons progress, which is in line with how he works himself up the career ladder and therefore deeper into the reefer lifestyle.