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They fall into two main groups902
| IDDSI level | Description |
|---|---|
| 0 | Thin (e.g. water) |
| 1 | Slightly thick |
| 2 | Mildly thick |
| 3 | Moderately thick (e.g. liquidised) |
| 4 | Extremely thick (e.g. pureed) |
| 5 | Minced and moist |
| 6 | Soft and bitesized |
| 7 | Regular / easy to chew |
Thickening agents can be used to modify the consistency of thin liquids to IDDSI levels 1-4. They are not medicines – many of them are classed as Food for Special Medical Purposes. Some brands are listed in UK drug tariffs, and are prescribable.902
The choice of thickening agent depends upon patient preference, as well as their living situation. Thickened fluids can be unpalatable, with starch-based products sometimes having a starch flavour and a grainy texture, whilst gum-based thickeners can be “slicker”.902 It is generally found that flavour and palatability deteriorate with increasing thickness regardless of the thickening agent used.902
Starch-thickened fluids can become thicker on standing, whilst gum-thickened fluids are slightly more stable.902 Some products can be stored in the fridge after preparation, whereas others have to be used immediately. Each product comes with its own scoop, and these are not interchangeable. The instructions for use also differ between products, with some products advising to add powder to liquid, and others to add liquid to powder.902 For patients with diabetes, or those on a ketogenic diet, it is important to consider the carbohydrate and sugar content of thickeners. If the patient lives with others who also require thickened fluids, such as in a care home, it is often practical to use one thickening agent for all residents.902
It is good practice to visually check the consistency of all fluids before they are taken to ensure that they are the appropriate thickness for the patient.901
As thickening agents are not intended for the administration of medications, there is little information about potential drug interactions; however it is known that some medications can interact with thickened fluids, altering the consistency of the fluid. For this reason adding medications to thickened fluids is generally the option of last resort and it is particularly important to visually assess the consistency of fluids to which medications have been added.
A well-reported interaction is that between macrogol laxatives and starch-thickened fluids, which results in a thin watery liquid. In 2019, the Institute for Safe Medication Practice in Canada shared an incident of this where the patient later died, possibly from aspiration.901 Thickening agents based on carob gum (also called locust bean gum) are also not suitable to be mixed with macrogol laxatives,901 and it is recommended that if any thickening agent is used for these medicines, it should be a xanthan gum-based agent, and the thickened liquid should be checked for consistency before being taken.902
Amitriptyline, tamoxifen, and verapamil have been documented as being incompatible with xanthan gum in products used to create extemporaneous liquids. It is not known whether adding the medications to thickened fluid immediately before administration would have a clinically significant interaction, but an alternative administration method should be used if possible.901
Xanthan, guar, and other gum-based agents are used in some modified-release drug formulations to slow drug dissolution,901 and some small studies have been done to assess the effect of thickening agents on drug dissolution.
An in vitro study of crushed tablets of amlodipine, atenolol, carbamazepine and warfarin, mixed with water, thickened water, honey, orange juice, yogurt and jam showed that drug dissolution was delayed in the thickened water compared to the other mixtures tested.901 Crushed atenolol was mixed with fluids thickened with five different agents, to three different consistencies (IDDSI 2, 3 and 4). Drug dissolution was not reduced at level 2, but at level 3 drug dissolution was reduced in the xanthan gum-thickened fluids, and at level 4 drug dissolution was reduced in both starch and xanthan gum-thickened fluids.901
Another study looked at various formulations of paracetamol (crushed tablets in water, effervescent tablets in water, elixir and suspension) all thickened with a xanthan gum and maltodextrin product to IDDSI level 4. The study found considerably delayed drug dissolution in all cases. Different methods of preparing the product were assessed (adding the medication to thickened fluid, adding thickening agent to the medication in water, adding water to a mixture of medication and thickener powder) but the delayed dissolution was found in all preparation methods.901
These studies demonstrate the possibility of delayed onset of effect when medications are administered in thickened fluids, and that this effect may become more pronounced with higher levels of viscosity.904
For medications with a wide therapeutic range, the effect of the thickening agent on the medication may not be clinically relevant, however it is important to monitor the effect of therapy, and to ensure that medications are given consistently by the same method. An alteration in the patient’s swallowing requirements (i.e. a change in viscosity) may have an effect on both effectiveness and / or toxicity of their medication.
Large-volume liquid medicine doses should be thickened by adding the thickening agent directly. The liquid should always be checked to ensure that the desired consistency has been obtained, and should be consumed immediately.904