yeah, it is (although it's not fatal, and neither is the second one, a muscle relaxant. the fatal agent is potassium chloride, which prevents nerve impulses and stops the heart.)
except it's not sodium pentothol, it's sodium pentothal, with an A. that's technically a trade name. the generic name is thiopental sodium.
sodium pentothal is colloquially known as "truth serum" because the sedation it causes makes it mentally easier to tell the truth than to lie. so it's been used in innumerable movies and even cartoons, although probably not so much in real life.
interestingly enough it also has some ability to protect cells against radiation, and can even protect against a fatal dose of radiation. making it an absolutely essential addition to your militia's bunker. back to that lethal injection thing, potassium iodide, a close relative of potassium chloride, also provides some protection against radiation (though it really only protects the thyroid).