Dr. Leibowitz (voiced by Gideon Evans) runs an impotence clinic, which the boys see advertised on television in the episode "Impotence". He is a heavy-set balding man with gray hair, wearing a white lab coat. The boys get the idea from the clinic's ad that he can help them "score". After eventually getting annoyed with their antics, he thinks "mankind needs me" and he gives them a bottle of saltpeter to discourage them from breeding. After taking it, the boys do not find the pictures they saw in the office nearly as stimulating.
Protagonists: Beavis / Cornholio / Old · Butt-Head / Old
Recurring Characters: Tom Anderson · Stewart Stevenson · Todd · Burger World Manager (2022) · Mrs. Stevenson · Mr. Stevenson · Lolita and Tanqueray · Maxi-Mart Clerk
Highland High School: David Van Driessen · Daria Morgendorffer · Principal McVicker · Coach Buzzcut · Spanish Teacher · Ms. Tress · School Nurse · Kimberly · Cassandra · Earl · Martin (thin) · Mrs. Dickey · Gina · Dreama · Dean Zunker · Punk Girl · Wilbur · Martin (gifted student) · JD · Glennis · Cody · Audrey · Mrs. Ortiz · Security Guard · Gym Teacher · Tommy · "Spare Me" Girls · Amy Asskowitz · Audrey's Friend · Dick Gaylord · School Psychologist · Ms. Jenkins · Jim the Substitute · Woodshop Teacher · Ryan · Librarian · Unnamed Female Teacher (Citizen Butt-Head)
Minor Characters: Marcie Anderson · Clark Cobb · Dave · Harry Sachz · Homeless Man · Madame Blavatsky · Redneck Lady · Crazy Farmer · Billy Bob · Slade · Psycho · Principal Brown · President Bill Clinton · Concert Girls · Hamid · Sterculius, the Roman God of Feces · St. Peter · Unnamed Police Officers · Mr. Manners · Frog · Harry Buddisker · Hiro · Mistress Cora Anthrax · Biker Lady · Fork Injury Guy · Peter Small · Killer · Second Killer · Ross and Harlan · Ellen · Keith · Case Worker · Rabid Ron · Lydia · Katya · Hairdresser · McVicker's Family · Beavis' Imaginary Children · Butt-Head's Imaginary Children · Gus Baker · Leroy · Fire · Jesus Christ · Ganesha · Buddha · Zeus · Bill Gates · Bill (Steamroller) · Linda · Joan · Mall Girls · Elevator Chick · Joe Adler · Suspicious Chick · Maya Kanigher · Megan · Old Woman (Couch-Fishing) · Cop (Couch-Fishing) · Cop (Pipe of Doom) · Cop (Screamers) · Mechanic · Slob · Priest · Rabbi · Jasmine · Captain Dick Jackman · Charlie the Angel · Misty Lou · Lucas · Dennis · Female Judge · Screaming Guy · Black Female Cop · Corvair Owner · Jim Baxley · Snake Woman · Urologist · Dr. Fisk · Dr. Leibowitz · Vet Receptionist · Wrong Mr. Anderson · Crystal · Sapphire · Gary · Shawna · Suspicious Chick · Debra Shea · Trish Burbee · Tiffany the Rubber Band Lady · Max Graham · Veterinarian · Victoria's Dad · Ric Aaron Joseph · Receptionist (Pumping Iron)
Movie Characters: Muddy Grimes · Dallas Grimes · ATF Agent Bork · ATF Agent Flemming · FBI Agent Hurly · Old Woman · Motley Crue Roadies · Judge · Serena Ryan · Jim Hartson · Mattison · Empress Beavis · Emperor Butt-Head
Smart Characters: Smart Beavis · Smart Butt-Head · Smart Daria · Smart McVicker · Smart Buzzcut · Smart Van Driessen · Smart Todd · Smart Judge · Smart Mrs. Stevenson
Mentioned Characters: Shirley Beavis · Tom Anderson's Son · Tom Anderson's Father