Phenobarbital may cause a severe allergic reaction. Stop taking phenobarbital and get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, eyes, lips, tongue, or throat.
Call your doctor at once if you have:
weak or shallow breathing;
unusual pain anywhere in your body (especially in the neck, shoulder, or arms);
a red blood cell disorder--pale skin, muscle weakness, diarrhea, weight loss, rapid heart rate, tongue swelling, numbness or tingling in your hands or feet, feeling short of breath; or
severe skin reaction--fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.
Side effects such as confusion, depression, or excitement may be more likely in older adults and those who are ill or debilitated.
Common side effects may include:
drowsiness, lack of energy;
dizziness or spinning sensation;
depressed mood;
feeling restless or excited (especially in children or older adults);
drunk feeling; or
"hangover" effect (drowsiness the day after taking phenobarbital).
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Phenobarbital side effects (more detail)
