Bisacodyl reduces serum digoxin levels to a small extent. Largeamounts of dietary fibre, guar gum and bulk-forming laxativescontaining ispaghula or psyllium appear to have no significant effect on absorption of digoxin. Single-dose studies show thatmacrogol 4000, a laxative polymer, reduces serum levels of digoxin.
Bisacodyl reduced mean serum digoxin levels of 11 healthy subjects by about 12%. When bisacodyl was taken 2 hrs before digoxin, serum digoxin levels were slightly raised, but not to a statistically significant extent (See reference number 1).
The fibre was given in this way to simulate conditions that might be encountered clinically (for example to reduce symptoms of diverticular disease) (See reference number 2)
Wheat bran 7.5 g twice daily caused a small 10 % reduction in plasma digoxin levels of 14 geriatric patients after 2 weeks, but there was no significant change after 4 weeks (See reference number 3). Bran fibre 11 g caused a 6 to 7 % reduction in absorption and steady-state serum levels of digoxin in 16 healthy subjects (See reference number 4). The cumulative urinary recovery of a single oral dose of digoxin in healthy subjects was reduced almost 20 % by 5 g of fibre,whereas 0.75 g of fibre had no effect (See reference number 5).
In 10 healthy subjects Guarem (95% guar gum) 5 g reduced peak serum levels of a single 500-microgram oral dose of digoxin by 21 % and AUC0-6 was reduced by 16%, but amount excreted in urine over 24 hrs was only minimally reduced (See reference number 6). Guar gum 18 g with a test meal did not affect steady-state plasma digoxin levels in 11 healthy subjects given digoxin 1mg on day 1,then 750 micrograms on day 2, then 500 micrograms daily for 3 days (See reference number 7).
A randomised, crossover study in 18 healthy subjects found that 20 g of macrogol 4000 daily over an 8-day period reduced maximum serum levels of a single 500-microgram dose of digoxin by 40%, and reduced AUC by 30%. Heart rate and PR interval were unchanged (See reference number 9). More study is needed to assess effects of this interaction on steady-state digoxin levels.
Not established. Digoxin can bind to some extent to fibre within gut (See reference number 10). However, in vitro studies (with bran, carrageenan, pectin, sodium pectinate, xylan and carboxymethylcellulose) have shown that most of binding is reversible (See reference number 11).
Information seems to be limited to these reports. The reduction in serum digoxin levels caused by bisacodyl is small, and not expected to be of clinical importance, and apparently preventable by giving bisacodyl 2 hrs before digoxin. Neither dietary fibre (bran), guar gum nor two bulk-forming laxatives (Vi-Siblin and Metamucil) have a clinically important effect on serum digoxin levels. No special precautions would appear to be necessary. The importance of interaction between digoxin and macrogol 4000 awaits further assessment, but on available evidence it would be prudent to be alert for need to increase digoxin dosage.
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