Contraindications
Fluorouracil is contraindicated in patients who are severely debilitated and in patients with bone marrow suppression due to either radiotherapy or chemotherapy. It is likewise contraindicated in pregnant or breastfeeding women. It should also be avoided in patients who do not have malignant illnesses. Adverse effects
Adverse effects by frequency include:

During systemic use
Common (> 1% frequency):
Uncommon (0.1–1% frequency):

Rare (< 0.1% frequency):

Anaphylaxis
Allergic reactions
Fever without signs of infectionDiarrhea is severe and may be dose-limiting and is exacerbated by co-treatment with calcium folinate. Neutropenia tends to peak about 9–14 days after beginning treatment. Thrombocytopenia tends to peak about 7–17 days after the beginning of treatment and tends to recover about 10 days after its peak. Cardiotoxicity is a fairly common side effect, usually manifesting as angina or symptoms associated with coronary artery spasm, but about 0.55% of those receiving the drug will develop life-threatening cardiotoxicity. Life-threatening cardiotoxicity includes: arrhythmias, ventricular tachycardia and cardiac arrest, secondary to transmural ischaemia. During topical use
Common (> 1% frequency):

Uncommon (0.1–1% frequency):

hyper- or hypopigmentation
Scarring

Neurological damage
The United States package insert warns that acute cerebellar syndrome has been observed following injection of fluorouracil and may persist after cessation of treatment. Symptoms include ataxia, nystagmus, and dysmetria. Potential overdose
There is very little difference between the minimum effective dose and maximum tolerated dose of 5-FU, and the drug exhibits marked individual pharmacokinetic variability. Therefore, an identical dose of 5-FU may result in a therapeutic response with acceptable toxicity in some patients and unacceptable and possibly life-threatening toxicity in others.
A fibrin ring granuloma may be present. Differential diagnosis
About 10% of people who present a clinical picture of infectious mononucleosis do not have an acute Epstein–Barr-virus infection. A differential diagnosis of acute infectious mononucleosis needs to take into consideration acute cytomegalovirus infection and Toxoplasma gondii infections. Because their management is much the same, it is not always helpful, or possible, to distinguish between Epstein–Barr-virus mononucleosis and cytomegalovirus infection. However, in pregnant women, differentiation of mononucleosis from toxoplasmosis is important, since it is associated with significant consequences for the fetus.Acute HIV infection can mimic signs similar to those of infectious mononucleosis, and tests should be performed for pregnant women for the same reason as toxoplasmosis.People with infectious mononucleosis are sometimes misdiagnosed with a streptococcal pharyngitis (because of the symptoms of fever, pharyngitis and adenopathy) and are given antibiotics such as ampicillin or amoxicillin as treatment.Other conditions from which to distinguish infectious mononucleosis include leukemia, tonsillitis, diphtheria, common cold and influenza (flu). Treatment
Infectious mononucleosis is generally self-limiting, so only symptomatic or supportive treatments are used. The need for rest and return to usual activities after the acute phase of the infection may reasonably be based on the persons general energy levels.