Sunset Sarsaparilla is a sweet carbonated sarsaparilla-based beverage that is found around the Mojave Wasteland. With its easy availability and low cost, Sunset Sarsaparilla is an excellent consumable that provides a high health regeneration effect. Thanks to these factors, it supplants the stimpak as a primary healing item in the Mojave.
In the company's "Silly Ol' Advisory", Festus provides a comical list of potential side effects, which include "kidney damage, nausea, digital numbness, anxiety, loss of visual acuity, dizziness, occasional nosebleeds, joint inflammation, tooth decay, sore throat, bronchitis, organ rupture and halitosis". In order for those effects to happen, Festus says you must drink "a helluva lot". However, you can have as many drinks as you want, and not suffer from those side effects in-game. Only while playing in hardcore mode does drinking Sunset Sarsaparilla have any negative effect – consumption produces mild dehydration.
The irradiated variant appears in the game as a normal bottle of Sunset Sarsaparilla, but it has absorbed a lot of radiation from sitting in the highly irradiated area of Camp Searchlight, thus giving it the negative effecting of dosing the user with 10 Rads upon consumption.
↑The Courier: "Silly Ol' Advisory." Festus: "While Sunset Sarsaparilla is perfectly safe, a recent independent study - whose validity is currently being challenged - revealed the following: Excessive ingestion of sarsaparilla can lead to deleterious effects including, but not limited to: kidney damage, nausea, digital numbness, anxiety, loss of visual acuity, dizziness, occasional nosebleeds, joint inflammation, tooth decay, sore throat, bronchitis, organ rupture, and halitosis. Note that you'd have to drink a heap of Sunset Sarsaparilla to match the quantities used in the study. How much, you ask? A lot. A whole helluva lot. In fact, you'd have to get full as a tick on Sunset Sarsaparilla to even come close. Anyway, thanks for stopping by, partner! And keep drinking Sunset Sarsaparilla!" (Festus' dialogue (Fallout: New Vegas))