Enemas viscosity

The volume of enemas may vary from a few millilitres (micro-enema) to more than 100 mL, mainly depending on the intended effect systemic or local. For large-volume enemas water is commonly used and a water-soluble form of the active substance is preferred. The solubility may be increased by addition of co-solvents, to be applied in small volume enemas. If a soluble active substance or an adequate co-solvent cannot be found, a suspension may be prepared. If this is also not an option, a lipophilic vehicle may be chosen. Choice of pH depends on the chosen form of the active substance and is important for the absorption. Excipients may be added to correct the osmotic value, to increase the viscosity, to prevent oxidation or for preservation. 

Excipients may be used to make an enema iso-osmotic, to increase the viscosity and sometimes for the wetting of... 

For a suspension enema, the addition of a viscosity enhancer may be necessary. Solutions sometimes have a viscosity enhancer added as well, based on the supposition that a prolonged residence time is beneficial for absorption [56] but published evidence of improved efficacy does not exist. On the contrary, a reduced absorption may be the result of reduced spreading in the gut. But probably the influence of viscosity on spreading is dwarfed by the influence of the intestinal pressure on spreading. 

When administering an enema, it is almost impossible to transfer all liquid to the rectum. A small amount will always remain in the bottle or giving tube. Small enema bottles are therefore filled with an excess of liquid. The required excess depends on the model of the micro-enema bottle and on the physical properties of the micro-enema liquid, in particular the viscosity. The residual volume, and thus the required excess, can be determined by weighing a bottle, filling it and emptying it by squeezing, after which it is reweighed. 

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