I use ibuprofen to help with Fibromyalgia pain and PEM from ME / CFS.
I've been prescribed desmopressin by my cardiologist for low bp, POTS and dysautonomia but I read that you aren't supposed to use these drugs together?
Why is this? Do you use them together? Is it a firm hard no or just use with caution? If so, what is the caution to be aware of? Thank you!
Surely if a medical professional gave you the drugs to be used together then it should be OK.... Ask them if you are unsure.. It might help ease your mind.
Did your cardiologist know about the Advil? If so you should be fine. Drugs.Com says
"Using desmopressin together with ibuprofen may increase the risk of developing water retention and a condition known as hyponatremia, which is caused by an abnormal decrease in blood sodium concentration. In severe cases, hyponatremia can lead to seizures, coma, and even death."
The may increase risk comment has two conditional statements and the can lead to has another conditional statement. The risk is moderate so I would think it is something that has been assessed and considered before being prescribed to you. If in doubt contact your primary physician or pharmacist.
Fibromyalgia and sweet syndrome together?
Does any body feel like the two sides of your body don't work together?
Fibromyalgia 
Feeling sick and tired of all this worry and feeling so poorly?
Me and water don't go well together. 
