Short answer: a guinea pig’s diet should be mostly unlimited grass hay, plus about 2 tablespoons of guinea pig pellets and fresh vegetables every day for vitamin C, according to the Merck Veterinary Manual. A hamster needs a complete hamster or rodent pellet as its staple, with seeds limited, treats under 10% of the diet and “a teaspoon or two” of fresh food a day, per Merck and PDSA.
Guinea pigs and hamsters have genuinely different dietary needs — despite both being small “pocket pets,” feeding them the same way is a common and avoidable mistake.
What Are Guinea Pig Diet Essentials?
- Hay (the majority of the diet): unlimited good-quality grass hay such as timothy or orchard grass. The RSPCA says it should be available at all times; it provides fiber and wears down teeth that grow throughout life
- Guinea pig pellets: a fresh daily portion of high-fiber, timothy-based pellets for adults — Merck suggests about 2 tablespoons a day. Alfalfa-based pellets suit growing or pregnant guinea pigs. VCA Animal Hospitals advises using pellets within 90 days of manufacture, because their vitamin C breaks down
- Fresh vegetables daily: the RSPCA’s everyday list includes bell peppers, cucumber, romaine or gem lettuce, celery and parsley. Merck advises limiting high-calcium greens such as kale, spinach and parsley because of bladder-stone risk, and the RSPCA lists broccoli, spinach and kale as a-few-times-a-week foods
- Vitamin C every day: guinea pigs can’t make it — see how much vitamin C guinea pigs need
- Fruit only as a treat: small amounts of apple or pear, because of the sugar
- Their own droppings: guinea pigs eat soft, moist droppings straight from their bottom, which the RSPCA notes are essential to their diet — this is normal
What Are Hamster Diet Essentials?
- A complete pellet as the staple: PDSA explains that pellets are uniform, so a hamster can’t pick out favorite bits the way it can with a muesli-style mix. The Merck owner guide says a rat or mouse pellet with 15–20% protein meets their needs, and VCA warns never to feed a seed-only diet because seeds are high in fat and poor in nutrition
- Treats under 10% of the diet: Merck’s cap, with seeds limited
- A little fresh food: PDSA suggests “a teaspoon or two a day” of vegetables, herbs and fruit, with fruit kept minimal
- Occasional protein: PDSA lists small pieces of boiled egg or mealworms as occasional treats
- Timothy hay: PDSA recommends it as a source of fiber that also helps keep teeth healthy
- Food as enrichment: scatter pellets or bury them in bedding instead of always using a bowl, as both PDSA and the RSPCA suggest
How Do Their Diets Compare?
| Guinea pig | Hamster | |
|---|---|---|
| Diet type | Herbivore | Omnivore |
| Staple food | Unlimited grass hay | Complete hamster/rodent pellet |
| Pellets | About 2 tbsp/day, guinea pig-specific | Measured daily portion; 15–20% protein |
| Fresh food | Daily vegetables, fruit as a rare treat | A teaspoon or two a day |
| Vitamin C | Essential every day | Not a special dietary requirement |
| Seeds and nuts | Avoid (VCA) | Limit; not a seed-only diet |
| Water | Continuous; check twice daily (RSPCA) | Constant; bottle checked daily (RSPCA) |
What Foods Should You Avoid?
For guinea pigs, the RSPCA lists iceberg lettuce, potato, garlic, avocado, coconut and citrus fruits as foods to avoid. VCA adds oats, seeds, nuts and dry cereals, and Merck warns that rabbit pellets don’t contain enough vitamin C for guinea pigs.
For hamsters, the RSPCA warns that grapes and rhubarb can be poisonous to rodents. Seed-heavy muesli mixes encourage selective eating, and sugary or salty human snacks don’t belong in either species’ bowl. When in doubt, ask an exotics vet before offering anything new.
How Should You Change or Introduce Foods?
Slowly. The RSPCA advises avoiding sudden diet changes for guinea pigs and introducing new foods gradually, and Merck notes that guinea pigs can be wary of the unfamiliar and may refuse to eat or drink after substantial diet changes — which is dangerous in a species that needs to eat constantly. In hamsters, Merck lists diet changes among the stresses that make diarrhea more likely; if you see a wet, dirty rear end, read our guide to wet tail in hamsters and call your vet.
How Do You Stop Hamster Food Hoards From Spoiling?
Hamsters carry food in their cheek pouches and stash it in their nest. Dry pellets in a dry stash are fine, but check daily for hoarded fresh food, egg or mealworms before they spoil — our guide to how often to clean a hamster cage covers how to do this without wrecking the nest.
Why Do Species-Specific Needs Matter?
The biggest feeding mistake with these two animals isn’t a wrong food choice — it’s assuming what works for one “pocket pet” works for the other. Guinea pigs are herbivores with a unique vitamin C requirement; hamsters are omnivores needing a completely different balance. Stick to a diet formulated for the species you have, and check with an exotics-experienced vet before making big changes. If you’re going away, remember guinea pigs need fresh food and hay topped up daily — see how long guinea pigs can be left alone.
This article is for informational purposes only and does not replace guidance from your veterinarian.
Sources
- Merck Veterinary Manual (Pet Owner Version) — Diet for a Guinea Pig, October 2025
- Merck Veterinary Manual — Housing and Nutrition of Guinea Pigs, May 2026
- Merck Veterinary Manual (Pet Owner Version) — Providing a Home for a Hamster, September 2026
- VCA Animal Hospitals — Feeding Guinea Pigs, August 2026
- VCA Animal Hospitals — Owning a Pet Hamster, February 2024
- RSPCA — Guinea pig diet
- RSPCA — Hamster diet
- PDSA — Hamsters as pets
