gisella,I am confused some people say miralax is safe and wont effect kidneys.i take Metamucil but that is slow to work.if I am constipated for 4 days what would be safe to take that wont produce diahhrea.I tried fiber one but doesn't work is miralax a few times a week safe on kidneys,i cant get a straight answer

Dear Howard,
Apologies for the delay in replying.  I hope your problem has resolved in the meantime.  Metamucil and Miralax have quite different mechanisms of action, and as such, their indications also differ.  Metamucil is a fibre supplement and has a rather slow onset of action.  It should be used to prevent constipation by 'supplementing' the diet with extra fibre.  It will not be effective in acute cases of constipation, and may, in fact, make matters worse in cases of obstruction.  Miralax, on the other hand, has a quicker onset of action and is designed to treat acute constipation.  Miralax contains polyethylene glycol (PEG), which belongs to the 'macrogol' laxatives.  This molecule is far too large to be absorbed from the gut and into the circulation.  Instead, it remains within the lumen of the gut and exerts an osmotic effect, meaning that it draws water into the lumen of the gut.  The extra fluid on board softens and aids in the passing of stool.

As the drug itself, at a molecular level, is too large to be absorbed systemically, it does not have a direct effect on the kidneys in its own right.  Theoretically, however, it could have an indirect action on the kidneys if excessive amounts of fluid and electrolytes within the body are lost via the gastrointestinal tract.  This is especially problematic for the elderly who are at a greater risk of dehydration and electrolyte imbalance, and for those with established renal impairment.  It is always important to mix the sachets with the volume of water recommended so as to prevent dehydration.  Many Macrogol products also include a supplement of electrolytes to minimise loss from the body.

From memory, Howard, your renal function was not all that bad, and so taking Miralax a few times a week should be fine.  Although we don't have this particular brand in Australia, we certainly do have others similar to it eg, Movicol.  This product is very popular, especially amongst the elderly. It is not uncommon to see it prescribed daily even.  My advice is to ensure you keep up your fluids.  You will most definitely need to cut back if you start to get recurring diarrhoea.  And as always, maintain a good rapport with your general physician.

Ideally, you should take Metamucil daily as a preventative measure.  You can always add the Miralax when needed.  You may find that after taking the Metamucil regularly, you won't need the Miralax as often.  Don't be afraid to take both because they serve two different functions.

I hope this helps.  


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Gisella Campanelli


I am able to answer questions relating to pharmaceuticals, therapeutic regimes and primary health care. This includes offering advice on drug indications, dosages, and disease state management. I can also identify side effects, drug interactions and contra-indications, and offer recommendations on ways to mitigate these. I can diagnose minor illnesses and suggest appropriate over-the-counter remedies and/or preventive healthcare tips. I can recognize cardinal symptoms which would otherwise require referral to a medical practitioner.


I am a registered pharmacist in Australia, and I have practiced in a hospital pharmacy for over thirteen years. My clinical specializations lie within the areas of psychiatry and general medicine (including gastroenterology, respiratory, endocrinology, neurology, infectious diseases, gerontology, dermatology). I self-managed the training program for pharmacy interns in preparation for their final registration exams, and I have worked for the Pharmacy Board of Australia as an examiner and exam writer.

I hold a Bachelor of Pharmacy from the Victorian College of Pharmacy, Monash University, and I am board-registered to practice within Australia. I also hold a Master's degree in an unrelated field (art conservation).

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