Zofran contains ondansetron, a medication that stops nausea and vomiting by blocking serotonin receptors in the body. Beyond that active ingredient, the full list of what’s in each Zofran product depends on the form you’re taking: standard tablets, orally disintegrating tablets (ODT), or the injectable version used in hospitals. Each contains a different set of inactive ingredients worth knowing about, especially if you have allergies or dietary sensitivities.
The Active Ingredient: Ondansetron
Every form of Zofran delivers the same active drug, ondansetron hydrochloride. It belongs to a class called serotonin-3 (5-HT3) receptor antagonists. Serotonin is a chemical messenger your body releases in response to many triggers, including chemotherapy drugs, anesthesia, and stomach infections. When serotonin binds to specific receptors on nerve endings in your gut, it sends a signal up the vagus nerve to your brainstem that triggers the urge to vomit.
Ondansetron works by physically blocking those receptors. It prevents serotonin released from cells lining the intestines from latching on and firing that signal. With the communication between gut and brain interrupted, nausea and vomiting are significantly reduced. This is why Zofran is commonly prescribed after surgery, during chemotherapy, and sometimes for severe morning sickness or stomach viruses.
What’s in the Standard Tablets
Zofran tablets come in 4 mg and 8 mg strengths. Aside from ondansetron, each film-coated tablet contains lactose, microcrystalline cellulose, pregelatinized starch, hypromellose, magnesium stearate, titanium dioxide, and triacetin. The 8 mg tablet also includes iron oxide yellow, which gives it a slightly different color from the 4 mg version.
Lactose is the most notable inactive ingredient here. If you have lactose intolerance, the small amount in a tablet is unlikely to cause digestive symptoms for most people, but it’s worth being aware of if you have a severe sensitivity. The other ingredients serve as binders (holding the tablet together), coatings (making it easier to swallow), and lubricants (preventing the tablet from sticking to manufacturing equipment).
What’s in the Orally Disintegrating Tablets
The ODT version is designed to dissolve on your tongue without water, which makes it especially useful when you’re actively nauseous and struggling to swallow a pill. These tablets are also available in 4 mg and 8 mg strengths and contain a noticeably different set of inactive ingredients: aspartame, gelatin, mannitol, methylparaben sodium, propylparaben sodium, and strawberry flavor.
A few of these are worth flagging. Aspartame is an artificial sweetener that contains phenylalanine. People with phenylketonuria (PKU), a rare genetic condition that prevents the body from processing phenylalanine, need to know this. The amount is very small (less than 0.03 mg per tablet), but the FDA requires it to be disclosed. Gelatin, derived from animal collagen, may be a concern if you follow a strict vegetarian, vegan, or certain religious diet. The parabens (methylparaben and propylparaben) are preservatives that a small number of people are sensitive to.
What’s in the Injectable Form
The injection is typically given in hospitals or infusion centers at a concentration of 2 mg per milliliter. Its inactive ingredients vary slightly depending on the packaging:
- Single-dose vials contain sodium chloride, citric acid, and sodium citrate as buffers, mixed in sterile water. No preservatives.
- Multidose vials contain the same base but add methylparaben and propylparaben as preservatives to keep the solution stable after the vial is opened multiple times.
- Premixed IV bags contain dextrose (a sugar) instead of sodium chloride, along with citric acid and sodium citrate buffers. These are also preservative-free.
If you’ve had reactions to parabens in the past, your care team can use a single-dose or premixed option that avoids them entirely.
How Your Body Processes Ondansetron
Once ondansetron enters your system, your liver breaks it down using several enzyme pathways simultaneously. No single enzyme dominates the process, which means the drug is less likely to be affected by genetic variations that slow down one particular enzyme. This is part of why ondansetron works reliably across a wide range of people. Most of the drug is cleared through the liver and eventually excreted by the kidneys.
Key Ingredients to Watch For
If you’re checking Zofran’s ingredients because of an allergy or sensitivity, here’s a quick summary of the most relevant items across formulations:
- Lactose: Present in the standard tablets. Not in the ODT or injectable forms.
- Aspartame (phenylalanine): Present only in the ODT tablets. Relevant for people with PKU.
- Gelatin: Present only in the ODT tablets. Relevant for dietary restrictions.
- Parabens: Present in the ODT tablets and the multidose injection vials. Not in standard tablets, single-dose vials, or premixed IV bags.
Because the three forms of Zofran have such different inactive ingredients, switching from one form to another can sometimes solve an ingredient sensitivity while still giving you the same anti-nausea benefit.