What is wrong with a little pain? Running hurts.
or Dialysis. Forever.
I'll take pain.
Blood flows into your kidneys in arteries. The arteries branch smaller and smaller until they are tiny little arterioles. These deliver blood to the microscopic unit in the kidney responsible for making urine, called the glomerulus. Those arterioles delivering blood to the glomerulus are called afferent arterioles. Those collecting the blood out the other side are called efferent arterioles. There are several hormonal mechanisms for making sure that the pressure head on the blood is sufficient so that you make appropriate urine even when you are dehydrated.
Ibuprofen and other NSAIDs mess with the hormonal control over the arterioles and essentially causes the blood to stop flowing to the glomerulus. This can cause a sharp decrease in urine and in oxygen delivery to the kidneys. The latter can kill off kidney cells in great number.
The effect of NSAIDS is mitigated by adequate hydration. If you are dehydrated, the protection is gone and the effect of the NSAIDS on the hormonal control over your afferent arterioles is gone. You can cause substantial and permanent kidney damage. Bad.
When glomerular filtration is limited, your body may try several mechanisms to fix the problem using other hormonal systems. This combined with reduced renal function can cause a dangerous increase in potassium and a drop in your sodium. Also bad.
You never know the day that your ibuprofen or other NSAID you take while running will cause renal failure, and it is not wholly dependent on hydration.
The pain of marathoning and training can be expected. Why do you feel the need to stop that pain? Because you don't like it? Marathoning is supposed to hurt. I'm not talking about the pathologic pain of injury; if you have that, address the injury (rather than hiding it under some drug). I see way too many people in my practice and in the world who want to run but don't want to hurt while running. Sadly, it does not always work that way. Don't risk your health to hide the pain. Learn to accept it, to feel it flow through you, to guide you and teach you. Pain is not your enemy.
A 2:35 marathoner friend who is a professional philosopher (really) once wrote this: "For me, racing is not about overcoming pain. It's about as running fast as I can. The pain is just the way the body speaks to you, a side phenomenon. It's silly to try to overcome it or attack it. We talk about the "pain" of running as if it were a single phenomenon. Actually, in running as in life there are many different sorts of pains, and one can learn to distinguish these pains in order to respond to them differently. It is very rare that generating the sensation of "pushing through it" is the appropriate response to pain. My sense is that generating that feeling is just layering one type of pain on another."
The standard nonprescription dose of motrin/ibuprofen/advil is 400 mg every 6 hours. Not 600 mg. Not 800. And certainly not 1000 mg. Sure, your doc may prescribe doses higher than 400 mg, but you are NOT your doctor (nor should you be, even if you are a doctor). The higher the dose, the greater the risk of injury to your kidneys, to your electrolytes and to your GI tract.