Many drugs that are prescribed for people with healthy kidneys have dosing adjustments for those with impaired renal function. This is mainly because the kidneys that usually clear these out from the body are not effective and giving the normal dose can cause unsafe levels of the drug to build up within the body.
So, for example, though the recommended dosage of the drug Levofloxacin, a potent antibiotic for people with normal renal function with certain kinds of bacterial infection is 500 mg every day, those on maintenance dialysis are advised to take only 250 mg every other day after the initial dose of 500 mg. This is simply to prevent the drug from accumulating in the body because the kidneys are not causing the drug to be excreted out of the body.
Dialysis does remove some drugs but only to a certain extent given the short nature of the treatments and the relatively infrequent sessions.
With me? Great! All good so far.
Now, what about people on nocturnal dialysis - those that are getting much longer, better and frequent treatments? One would expect that the dosage for them (rather, us) would be close to those with healthy kidneys.
Not so simple!
There are a lot of factors that come into play here. The molecular weight of the drug is one of them. The ease with which substances are dialyzed out during dialysis is inversely proportional to their size or molecular weight. The protein binding of a drug is also important. The greater the protein binding of a drug, the lesser the tendency for it to dialyze out during a dialysis session.
It is therefore very difficult to come to any conclusion about the dose in those on nocturnal dialysis simply because there just haven't been enough studies on this subject. Well, you would expect that, wouldn't you? There are such small numbers of people doing this therapy that researchers probably feel that it is not worth the effort!
This whole question came up when I was advised by my family doctor to take the regular dose of Levofloxacin along with a cough syrup to battle my horrible cough. As is my habit, I looked it up on the internet and sure enough found that there was an altered dosage recommended for those on dialysis.
I posted this question to the good Dr. John Agar of Australia on the Home Dialysis Central forums. He said the same thing. We don't know for sure but he would recommend going with the hemodialysis dosage.
But in a theoretical sense, it is still an open question!
There is a great article for the medically inclined available here on the subject.