BULKING: Why 10mg tablets aren’t just 10mg

ABSTRACT:
Ever wonder why there is so much other crap in our tablets? 
Although the active ingredient in our tablets come as 1mg, 5mg or 50mg, the tablet itself - a sometimes powdery mess - must be a minimum of 120mg in total weight to have an accurate dose.
Today, I'm specifically looking at the "bulking agents" (an excipient or 'other' ingredient) found in medicines.  

METHODOLOGY: 
- Learning about dilutions using textbook: 
Introduction to Pharmaceutical Calculations by Judith A. Rees et al. 
- Following up fact found in the textbook using: 
The Pharmaceutical Codex

DISCUSSION AND RESULTS:
While trying to get my head around different types of dilutions and why they exist in pharmacy, I learnt that we aren't able to accurately measure substances that weigh under 100mg. 
In the Introduction to Pharmaceutical Calculations book, a reference was made to The Pharmaceutical Codex. 
The codex states that powders must be a minimum of 120mg in formulations. 
The reason is to maintain an error rate of less than 5% - 120mg is around the weight where our measuring instruments become less reliable. 

This got me wondering about all the tablets out there where the active ingredient is less than 120mg - like cetirizine, your classic OTC medicine that is only 10mg. 
So how is this tablet made accurately and safely if we can't make something that is less than 120mg?

DILUTIONS and triturations with bulking agents!

We usually see "bulking agent" written in the excipients (other ingredients) section of a medicine's information leaflet. It sounds like the tablet has been hitting the gym but it's actually just an inactive ingredient that brings the weight of the tablet up to 120mg. 
Of course, there are a lot of other functions of bulking agents (such as making a manageable sized tablet to take, or to bind the ingredients together), but this post will focus on the crazy new fact/realisation that they add to a tablet's weight for a reason.

As an aside - technicianlly, in this topic the bulking agent is called the diluent (the bulking agent literally dilutes the cetirizine and in doing so, adds weight to the tablet).
However, I'm only going to refer to it here as the bulking agent because that is how it's listed in the information leaflet and there's a 1000000% chance a patient will refer to it as that in a query, and not a diluent. 

One method we can use to create safe tablets, is to make enough of the active ingredient and a bulking agent for multiple tablets. 
This larger amount of final product can be measured accurately and smaller portions can be taken to create single tablets - or different doses - safely.

For the cetirizine example: 

We are going to make 100 x 10mg tablets. 
This means we need 1000mg of cetirizine. 
Each 10mg tablet will also need 110mg of bulking agent to bring it to the 120mg that can be measured accurately.
This means we will need 11,000mg of bulking agent for 100 tablets. 
If we mix the active ingredient with the bulking agent, we will have 12000mg of product at a ratio of 1/11 (for every 1 part of cetirizine we have 11 parts of bulking agent). 
Smaller amounts of this mixture will have the same ratio of cetirizine to bulking agent, so, if we measure out 120mg from this mixture, we should have 1 tablet that contains 10mg of cetirizine. 

This is incredibly important in making sure that the dose of a tablet is correct. 
If we made a tablet that had 5% too much of an active ingredient, the patient could suffer more from side effects. 
If we made a tablet that had 5% too little of an active ingredient, the patient's condition might not be treated as effectively as intended; and their health could deteriorate. 

LOGICAL CONCLUSIONS:
Ingredients in medicines that are smaller than 120mg cannot be accurately measured. 
Therefore, tablets are often 'bulked up' to 120mg with inactive ingredients. This weight maintains an accuracy error rate of less than 5%.
To achieve this, larger amounts of each ingredient, in excess of 120mg, are produced and mixed together at the required ratio/concentration. Fractions are then taken from this mixture to create single tablets with a reliable dose. 
The mixture can also be diluted further to achieve different doses. 
Using weights that can be reliably measured ensures a safer product for patients.