Your pet is being treated with insulin to control diabetes.  It is important for you to understand how insulin is labeled and how different insulin syringes are labeled in order to be able to accurately administer this medication. 

There are many different types of insulin on the market.  Your veterinarian has selected the type of insulin, which is best suited for your pet.  Insulins are all named with two parts to their name; the type of insulin and the strength of the insulin.  The strength of the insulin, refers to how many Units of insulin are present in each milliliter (or CC) of the insulin solution.  This part of the name is generally listed as follows:

U-____   where the blank is filled with the strength of the insulin.  Therefore:

  • U-100 insulin has 100 units of active insulin in each mL of liquid.
  • U-40 insulin has 40 units of insulin in each milliliter (mL) of liquid. 

This means that the same volume (liquid amount) of a U-100 insulin has 2.5 times more insulin in it than a U-40 insulin.  Or, the U-100 insulin is 2.5 times stronger than the U-40 insulin.  To be more specific, if we take 1/10 of an ML of U-100 insulin, we get 10 units of insulin (100 units/ml  x 0.1 ml= 10 units).  If we take 1/10th of an ml of U-40 insulin, we get  4 units of insulin (40 units/ml x 0.1 ml = 4 units).

In order to make dosing of insulin both easy and accurate, manufacturers make syringes which are specifically designed for each insulin.  The individual lines on each syringe represents 1 unit of insulin so that you can easily select the right amount of units for your pet.  So, if you have U-100 insulin, it is best to use U-100 syringes, and if you use U-40 insulin it is best to use U-40 syringes.   U-100 syringes are the most readily available in most pharmacies, however, we can special order U-40 syringes. 

If you use U-100 insulin with U-100 syringes, you simply count the number of lines on the syringe and match that to the number of units.

If you use a U-40 insulin and the U-40 syringes that are provided with that insulin, you fill the syringe in the normal manner and don’t need to do any conversions.

Some people who use U-40 insulin often prefer to use the U-100 syringes because they are readily available at the local pharmacy and they have a very thin needle.

But if you use a U-40 insulin and a U-100 syringe, you must correct for the difference in the strength of the insulin when you fill the syringe.

The following information explains how to fill a syringe using a U-40 insulin and a U-100 syringe. 

To get a specific number of units of U-40 insulin using a U-100 syringe, you must multiply by 2.5, and fill the syringe to that mark.  For example, to get 5 units of U-40 insulin using a U-100 syringe you must fill the syringe to the 12.5 unit mark on the barrel of the syringe.  The chart below can be used as a reference.  You can print this chart and use it for reference, or create a chart of your own….just be sure you understand the math.

If you do not understand this concept, please discuss it with your veterinarian.  It is very important that you understand how many actual units of insulin you are giving and why you must fill the syringe this way. If you tell your vet that you give 12.5 “units” when you really mean that you fill the syringe to the 12.5 unit mark, you are actually giving 5 units of insulin. For your pet’s health and safety, both you and any vet you talk to must know how many units of insulin your pet is getting.

This concept can also be used when dealing with diluted insulins.  A U-100 insulin that has been diluted is no longer a U-100 insulin.  The dilution has made it weaker.  So, the mark that you fill the syringe to is not the actual number of units of insulin you are giving.  Be sure you understand how your pet’s insulin was diluted and how many units of insulin you are giving.  If your pet’s insulin was a U-100 insulin that was diluted to be half of its original strength, it is now a U-50 insulin. This means that when you fill the standard U-100 syringe to the 5 unit mark, you are really giving 2.5 units of insulin.  

If you are using a diluted insulin, the insulin bottle should be properly labeled to show that the insulin was diluted. Be sure to have the person who diluted your insulin write down exactly what they did.  Have them write down what the strength (U-#) of the original insulin was, and how much “original” insulin and how much diluent they used to make the new, diluted insulin.  Even if your vet did the dilution, you should have this information in your records at home.  There may be a time when you have an emergency and must take your pet to a different vet.  That emergency vet will need to know exactly how the insulin was diluted.

A dilution tip: some owners have reported problems with insulins that were diluted using saline solution.  Sometimes there are problems with saline-diluted insulins not working properly.  Also, the injections of saline-diluted insulins can cause discomfort for your pet.  The  proper diluent (the liquid that is used to make that specific type of insulin) can be obtained from the company that manufactured the insulin. It is usually free.

Conversion chart for using a U-40 insulin with a  U-100 syringe

To get this many
units of
U-40 insulin

Fill a
U-100 syringe
to this mark

To get this many
units of
U-40 insulin

Fill a
U-100 syringe
to this mark

0.25

0.6

6.25

15.6

0.50

1.3

6.50

16.3

0.75

1.9

6.75

16.9

1.00

2.5

7.00

17.5

1.25

3.1

7.25

18.1

1.50

3.8

7.50

18.8

1.75

4.4

7.75

19.4

2.00

5.0

8.00

20.0

2.25

5.6

8.25

20.6

2.50

6.3

8.50

21.3

2.75

6.9

8.75

21.9

3.00

7.5

9.00

22.5

3.25

8.1

9.25

23.1

3.50

8.8

9.50

23.8

3.75

9.4

9.75

24.4

4.00

10.0

10.00

25.0

4.25

10.6

10.25

25.6

4.50

11.3

10.50

26.3

4.75

11.9

10.75

26.9

5.00

12.5

11.00

27.5

5.25

13.1

11.25

28.1

5.50

13.8

11.50

28.8

5.75

14.4

11.75

29.4

6.00

15.0

12.00

30.0