Excipients – what are they?


Excipients – what are they?


An excipient is an inactive ingredient of a medicine which is added in the formulation for a specific purpose.

Excipients must be listed on the medicine’s labelling. They can also be found in the patient information leaflet within the packaging.

Excipients are used for various reasons such as fillers, binders, adhesives, diluents, disintegrants, glidants, lubricants, flavourings, tablet coatings, colourants, preservatives and sweeteners. Here is a list of why some of them are used (this is only a few of the many used):


Binders such as gelatin, glucose, lactose, cellulose derivatives (methyl cellulose, ethyl cellulose, hydroxy propylmethyl cellulose, hydroxy propyl cellulose), starch, poly vinyl pyrrolidone (Povidone), sodium alginate, carboxymethylcellulose, acacia, etc. These bind the tablet ingredients together giving form and strength.

Diluents such as lactose, dextrin, glucose, sucrose, micro crystalline cellulose, sorbitol, dibasic calcium phosphate dehydrate, calcium sulphate dehydrate, etc.

These provide bulk and enable accurate dosing of potent ingredients.

Disintegrants such as starch, cellulose derivatives and alginates and crospovidone. These aid dispersion of the tablet in the gastrointestinal tract, releasing the active ingredient and increasing the surface area for dissolution.

Glidants such as colloidal anhydrous silicon and other silica compounds, cornstarch, talc, etc. These improve the flow of powders during tablet manufacturing by reducing friction and adhesion.

Lubricants such as stearic acid and its salts (e.g. magnesium  stearate), talc and paraffin. These have similar actions to glidants, however, they may slow

disintegration and dissolution. The properties of glidants and lubricants differ, although some compounds, such as starch and talc, have both actions.

Tablet coatings such as sugar (sucrose) and polymers. Polymers that are insoluble in acid, e.g. cellulose acetate phthalate, are used for enteric coatings to delay release of the active ingredient.

Colourants such as synthetic dyes and natural colours. These are used to improve acceptability to patients, aid identification and prevent counterfeiting as well as to increase stability of light sensitive drugs.

Excipients are supposed to be inert but often they have been found to cause problems for patients.


Do they matter to us?

Thyroid UK is informed on a daily basis that patients have reactions to certain brands of levothyroxine, liothyronine and natural desiccated thyroid medicines. In most cases, when they change to a brand with different excipients, their symptoms disappear.

The case of Merck changing lactose to mannitol in their levothyroxine’ causing thousands of patients to become ill, is an important one to remember – https://www.bmj.com/content/360/bmj.k714

This article in the journal, General Cardiology, reported on a study “Inactive Ingredient in Oral Medications” which explains about excipients and the most common adverse reactions:


There doesn’t appear to be an up to date list of excipients but this Handbook of Pharmaceutical Excipients gives a lot of information:  https://www.nps.org.au/australian-prescriber/articles/pharmaceutical-excipients-where-do-we-begin

This 2019 article – The Excipients between Effects and the Side Effects by Mohammed Abdul-Mutalib Abdul-Bari – explains a lot about excipients and concludes that:

“As shown above, excipients have been used for a long time because of its benefit in enhancing drug effect, performance, solubility and other beneficial properties that make the drug more effective and safe.

On the other hand, the potential adverse effect of the excipients should not be underestimated and a periodic review and follow up of side effects that may appear and occur after a drug administration should not be limited to the active medication in the drug formula but also the excipient should be taken into account

We all, therefore, need to be aware that it could be the excipients that might be causing any symptoms when we start on a new brand of thyroid medication and not the active ingredients.”

If you have new symptoms or a return of your hypothyroid symptoms and you have had a change of brand/generic, do consider that your symptoms may be due to adverse reactions to the excipients and consider asking to be given a brand/generic that suits you.


Thyroid UK relies on donations so that we can continue to support and campaign for people with thyroid disease and related disorders.  If you have found our information helpful, please do think about donating or becoming a member.



Become a member



Date created:  10/04/20
Review date:   10/04/22