Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor immediately if any of the following side effects occur:
More common
Ankle, knee, or great toe joint pain
joint stiffness or swelling
rash
rash with flat lesions or small raised lesions on the skin
Rare
Agitation
ammonia-like breath odor
anxiety
bleeding gums
blistering, peeling, or loosening of the skin
blood in the urine or stools
bloody nose
bloody or black, tarry stools
blue or pale skin
bruising
changes in skin color
chest pain or discomfort
chest pain, possibly moving to the left arm, neck, or shoulder
chills
clay-colored stools
cloudy urine
confusion
constipation
cough
coughing up blood
cracks in the skin
dark urine
decreased awareness or responsiveness
decreased urine output
depression
diarrhea
difficulty with breathing
dizziness
drowsiness
dry mouth
feeling of warmth or heat
fever
flushing or redness of the skin, especially on the face and neck
general feeling of discomfort or illness
headache
hives or welts, itching, skin rash
hoarseness
hostility
incoherent speech
increased urination
irritability
joint or muscle pain
large, flat, blue or purplish patches in the skin
light-colored stools
loss of appetite
loss of consciousness
loss of heat from the body
lower back or side pain
metallic taste
muscle twitching
muscle weakness
nausea
noisy breathing
pain, tenderness, or swelling of the foot or leg
painful or difficult urination
pinpoint red or purple spots on the skin
rapid weight gain
red, irritated eyes
red, swollen skin
redness, soreness, or itching skin
right upper abdominal or stomach pain and fullness
scaly skin
seizures
severe sleepiness
severe stomach pain
slow or irregular heartbeat
sore throat
sores, ulcers, or white spots on the lips or in the mouth
sores, welting, or blisters
stomach pain
sweating
swelling of the face, ankles, hands, or lower legs
swollen or painful glands
swollen, painful, or tender lymph glands in the neck, armpit, or groin
thirst
tightness in the chest
unpleasant breath odor
unusual bleeding or bruising
unusual drowsiness, dullness, tiredness, weakness, or feeling of sluggishness
unusual weight gain or loss
vomiting
vomiting of blood or material that looks like coffee grounds
yellow eyes or skin
Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Rare
Bad, unusual, or unpleasant (after) taste
blindness
blue-yellow color blindness
blurred vision
body aches or pain
burning feeling in the chest or stomach
burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
burning, dry, or itching eyes
change in taste
change in vision
congestion
continuing ringing or buzzing or other unexplained noise in the ears
decreased interest in sexual intercourse
decreased vision
difficulty with moving
discharge or excessive tearing
feeling of constant movement of self or surroundings
hair loss or thinning of the hair
hearing loss
impaired vision
inability to have or keep an erection
indigestion
lack or loss of strength
loss in sexual ability, desire, drive, or performance
loss of appetite
loss of memory
multiple swollen and inflamed skin lesions
muscle pain, cramps, or stiffness
muscular pain, tenderness, wasting, or weakness
problems with memory
redness, pain, or swelling of the eye, eyelid, or inner lining of the eyelid
runny nose
sensation of spinning
sensitivity to light
sleepiness or unusual drowsiness
sleeplessness
sneezing
stomach upset
stuffy nose
sweating
swelling of the breasts or breast soreness in both females and males
swelling of the salivary glands
swelling or inflammation of the mouth
tearing
tender, swollen glands in the neck
tenderness in the stomach area
throbbing pain
trouble getting pregnant
trouble with sleeping
trouble with swallowing
unsteadiness or awkwardness
voice changes
weakness in the arms, hands, legs, or feet
weight loss
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
 Zyloprim side effects (more detail)
