Hives are a severe allergic reaction, you must take him to the ER ASAP.
MiraLax Side Effects
Seek medical attention right away if any of these SEVERE side effects occur when using MiraLax Powder for Oral Solution:
Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue).
Please note - some side effects for MiraLax may not be reported. Always consult your doctor or health care specialist for medical advice. You may also report side effects to the FDA 1-800-FDA-1088 (1-800-332-1088).
Side Effects of MiraLax - for the Consumer
MiraLax Powder for Oral Solution
All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using MiraLax Powder for Oral Solution:
Cramps; diarrhea; excessive or frequent bowel movements; gas; nausea; stomach bloating.
Seek medical attention right away if any of these SEVERE side effects occur when using MiraLax Powder for Oral Solution:
Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue).
This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.
MiraLax Powder Packets for Oral Solution
All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using MiraLax Powder Packets for Oral Solution:
Cramps; diarrhea; excessive or frequent bowel movements; gas; nausea; stomach bloating.
Seek medical attention right away if any of these SEVERE side effects occur when using MiraLax Powder Packets for Oral Solution:
Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue).
This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.
https://www.drugs.com/sfx/miralax-side-effects.html
All the best,