From the nervous system and sensory organs: dysphoria, headache, dizziness, vertigo, drowsiness, weakness, paresthesia.
Rare: toxic psychosis or short cramps, usually when used in high doses. Seizures were noted in patients with predisposing diseases (epilepsy, cerebral atherosclerosis).
In children and infants who received nalidixic acid in therapeutic doses, there was sometimes an increase in intracranial pressure with protrusion of the fontanel, edema of the optic nerve disk and headache.
Several cases of paralysis of a pair of cranial nerves were noted. The mechanism of these reactions is unknown, signs and symptoms of them usually quickly pass without consequences after the withdrawal of drugs.
Rarely were there subjective visual impairments without objective disturbances, such as excessive brightness of color perception, violation of color perception,difficulty focusing and reducing visual acuity, diplopia (usually observed during the first days of treatment and quickly disappeared with a lower dose or abolishment of drugs).
On the part of the intestine: dyspeptic phenomena, nausea, vomiting, diarrhea, gastralgia, bleeding from the digestive tract, cholestasis.
Allergic reactions: skin rash, itching, angioedema.
Others: myalgia; with prolonged use, thrombotic and leukopenia, hemolytic anemia, photosensitivity, convulsions are possible.