Tamiflu contains one active ingredient, oseltamivir phosphate, which is an antiviral drug that works against influenza A and B viruses. The rest of the formula is made up of inactive ingredients that vary depending on whether you’re taking the capsule or the liquid suspension. Here’s a full breakdown of everything inside both forms.
The Active Ingredient: Oseltamivir Phosphate
Oseltamivir phosphate is technically a “prodrug,” meaning it doesn’t become active until your liver processes it. Once swallowed, your liver converts it into its active form, oseltamivir carboxylate, which then gets to work against the flu virus.
The drug targets a protein called neuraminidase that sits on the surface of flu viruses. Normally, neuraminidase helps newly formed virus particles break free from an infected cell so they can spread to other cells. Oseltamivir blocks that protein, essentially trapping new virus copies on the cell surface and preventing them from spreading further through your body. This is why Tamiflu works best when taken within 48 hours of symptom onset: the sooner you limit viral spread, the shorter and milder the illness tends to be.
What’s in the Capsules
Tamiflu capsules come in three strengths: 30 mg, 45 mg, and 75 mg. All three contain the same core fillers and binders inside the capsule, but the shells are colored differently so pharmacists and patients can tell them apart.
The powder inside every capsule contains:
- Pregelatinized starch, a filler that helps the powder flow and compress properly
- Povidone K30, a binder that holds the powder together
- Croscarmellose sodium, which helps the capsule contents break apart and dissolve in your stomach
- Sodium stearyl fumarate, a lubricant used during manufacturing
- Talc, another flow agent
The capsule shells themselves are made from gelatin and colored with iron oxides and titanium dioxide. The 30 mg capsule uses red and yellow iron oxide for a light yellow appearance. The 45 mg capsule uses black iron oxide for a gray tone. The 75 mg capsule uses all three iron oxides (black, red, and yellow). Each capsule is printed with blue ink containing FD&C Blue No. 2.
What’s in the Liquid Suspension
The oral suspension is a powder that gets mixed with water at the pharmacy, producing a liquid with a concentration of 12 mg per milliliter. It’s flavored with a tutti-frutti flavoring to make it easier for children to take. Beyond the active ingredient, the liquid contains:
- Sorbitol, a sugar alcohol used as a sweetener
- Saccharin sodium, an artificial sweetener
- Xanthan gum, a thickener that keeps the drug evenly distributed in the liquid
- Sodium benzoate, a preservative
- Monosodium citrate, a buffering agent that stabilizes acidity
- Colloidal silicon dioxide, which prevents clumping
- Titanium dioxide, a whitening agent
Once mixed, the suspension needs to be refrigerated and used within 17 days. If stored at room temperature, the window shrinks to 10 days.
Sorbitol Content and Fructose Intolerance
The liquid formulation contains a notable amount of sorbitol. A single 75 mg dose of the suspension delivers about 2 grams of sorbitol, and a full bottle contains roughly 11 grams. For most people this is harmless, but for anyone with hereditary fructose intolerance, that amount exceeds the recommended daily limit and can cause digestive symptoms like bloating and diarrhea. The capsule form does not contain sorbitol, so it’s a better option for anyone sensitive to sugar alcohols.
Gelatin and Dietary Restrictions
All three capsule strengths use gelatin in their shells. The FDA labeling does not specify whether the gelatin comes from a bovine (cow) or porcine (pig) source. If you follow a vegetarian, vegan, halal, or kosher diet and need to avoid certain types of gelatin, the liquid suspension is gelatin-free. You can also ask your pharmacist to confirm the gelatin source with the manufacturer directly.
Neither the capsules nor the suspension contain lactose. The labeling does not list any gluten-containing ingredients, though neither formulation carries a formal gluten-free certification.
Generic Versions
The FDA has approved generic versions of oseltamivir phosphate capsules in all three strengths. Generic formulations contain the same active ingredient at the same dose, but inactive ingredients like fillers, dyes, and binders can differ from the brand-name version. If you have a known sensitivity to a specific inactive ingredient, compare the generic’s label to the brand-name list before switching.