As with the majority of medications, a patient may have an allergy to Zofran. In cases such as this, the patient should immediately discontinue use of the drug and notify medical personnel if the following symptoms are present: swelling in the facial region or tongue, hives, or trouble with breathing. The most common adverse reactions are mild and may include diarrhea, constipation, fatigue, headaches, fever, or dizziness. If these reactions are chronic or pronounced, your local doctor may need to be contacted to advise if a change in Zofran dosage level is needed. More serious side effects that will definitely require intervention are as follows: vision impairment, breathing impairment, lowered heart rate, shivering, anxiety, decreased urination, agitation, or a feeling that the patient may faint.