The scores
Hill’s Prescription Diet i/d: B (76/100)
Hill’s Science Diet: B (76/100)
Both land at B/76 — in fact their top four ingredients are identical, so the rubric scores them the same. The two differ only at the margins, and both follow the same Hill’s pattern: a strong protein start followed by a grain and corn middle section, with the Rx version leaning on more carefully engineered digestibility.
How the ingredients compare
Here are the top five ingredients side by side:
Prescription Diet i/d: Chicken, Cracked Pearled Barley, Brown Rice, Brewers Rice, Whole Grain Corn
Science Diet: Chicken, Cracked Pearled Barley, Brown Rice, Brewers Rice, Whole Grain Wheat
The first four ingredients are identical — same chicken, same barley, same rice duo. The split happens at position five: Rx i/d uses whole grain corn, while Science Diet uses whole grain wheat. From there, Science Diet piles on more fillers — corn protein meal, chicken liver flavor, soybean meal — while Rx i/d keeps a cleaner supporting cast with egg product, dried beet pulp, and flaxseed.
Where Hill’s Prescription Diet i/d is cleaner
The biggest difference is what Rx i/d doesn’t include. Science Diet loads up with wheat, soybean meal, and corn protein meal — three plant protein fillers that inflate the protein number without delivering the amino acid profile of actual meat. Rx i/d avoids wheat and soybean entirely, and while it still has corn protein meal, the overall filler ratio is noticeably lower.
Rx i/d also includes egg product, a highly digestible animal protein source that Science Diet lacks. Flaxseed appears in both formulas, but Rx i/d adds cranberries — a functional ingredient with urinary tract benefits. The formula was engineered for digestive gentleness, and that design philosophy shows in the cleaner ingredient deck.
Science Diet uses both chicken liver flavor and pork liver flavor — two palatability boosters designed to make dogs eat a formula they might otherwise reject. Rx i/d uses only chicken liver flavor. When a food needs two separate flavor additives to get dogs interested, it raises questions about how appealing the base formula actually is. Shop on Amazon →
Where Hill’s Science Diet holds its own
Science Diet has one genuine advantage: fructooligosaccharides (FOS), a prebiotic fiber that feeds beneficial gut bacteria. It’s ironic — the standard retail formula includes a digestive support ingredient that the prescription digestive diet omits. Rx i/d was designed for dogs with active GI issues, yet it contains no probiotics and no prebiotics. For a digestive-focused formula, that’s a surprising gap.
Science Diet is also available without a veterinary prescription and costs significantly less. If your dog doesn’t have diagnosed digestive issues, Rx i/d isn’t just more expensive — it’s a therapeutic diet being used off-label. Price matters, and Science Diet’s wider availability is a practical advantage for everyday feeding. Shop on Amazon →
The bottom line
Hill’s Prescription Diet i/d (B/76) ties Hill’s Science Diet (B/76) on our rubric — the two share their top four ingredients, and i/d’s slightly cleaner filler profile is offset by the prebiotic Science Diet carries and i/d lacks. The comparison comes with an asterisk the size of a dinner plate: Rx i/d is a veterinary-prescribed therapeutic diet, not a premium upgrade you grab off the shelf.
If your vet has recommended Rx i/d for your dog’s digestive issues, the B/76 score confirms it’s a solid option on ingredient quality. If you’re simply looking for something better than Science Diet for a healthy dog, skip the prescription aisle entirely — brands like Blue Buffalo (B/75) sit right alongside and Wellness (B/78) edges it out, at non-prescription prices.