So well, here is some information I have found on rx list about the over dosage of naproxen, hope this would help.
A significant naproxen overdose can be distinguished by the following symptoms: drowsiness, heartburn, lethargy, nausea, epigastric pains, dizziness, renal dysfunction, abdominal discomfort, vomiting, hypoprothrombinemia, indigestion, transient alternations in liver function, apnea, metabolic acidosis and/or disorientation. Besides, there are also chances that gastrointestinal bleeding may occur. Other symptoms that may occur but they are pretty rare as the chances of their occurrence is pretty low: acute renal failure, hypertension, respiratory depression as well as coma. Moreover, there have also been reported some anaphylactic reactions with the therapeutic ingestions of NSAID’s so it is believed that it may also occur followed by an overdose of naproxen. Due to the fact that naproxen sodium can be quickly absorbed by the body, early and high blood levels need to be anticipated. It is reported that there are a few patients that have also experienced convulsions, however it has still not been established whether these are drug related effects or not. it has also not been established what is the dose of the drug for it to be life threatening.
So well, as you can see it has not been said what amount it is being considered overdose. However, one sure thing is that 8 pills is surely a supra therapeutic dose, however, as it is being said, mostly it depends on your sister’s health condition to determine whether something might happen to her or no. I personally, have taken either 6 or 7 pills of naproxen of 200 mg each at one single time (stupid, I know) but the point is that I have had zero side effects! so, that is why I can presume that in case your sister is in a good health condition (especially if she is not having compromised kidney function) then she needs to be fine, except the fact that she might have a little bit of stomach discomfort that would disappear by itself with time and/or some heartburn.
Anyway, some time has already passed, how is your sister feeling? What have you done to help her? is she alright?