How Do Pharmacists Count 'Day Supply' For Controlled Substances?

In our latest question and answer, the pharmacist discusses how 'day supply' is calculated for controlled substance prescriptions.


I picked up a prescription for ten 72-hour transdermal fentanyl patches (a 30-day supply) on February 19, 2019 (it was filled on 2.18.19, I picked it up a day after). The pharmacy’s policy on controlled substance refill pick up is 28 days (they allow pick up 2 days early). I called the pharmacy to make sure my Rxs would be ready on March 18 and was informed the transdermal patches would not be filled until tomorrow (March 19, 2019). I asked why and the pharmacist stated the store policy, 28 days/2 days early, and said that if I had wanted my prescription filled early (today, March 18, 2019) he would need approval from my doctor. How do pharmacists count day supply? Shouldn't I be able to fill these?

Asked by Mia On Jun 29, 2022

Answered by
Medical Content Reviewed By HelloPharmacist Staff

Published Jun 30, 2022
Last updated Jun 30, 2022

Key points

  • In general, the day you pick up your controlled substance prescription is not counted as 'day one' when calculating your 'day supply'. Instead, the next day is considered 'one day'.
  • For example, picking up a prescription on March 1st, March 2nd would be 'day one', not 'day two'.
  • Using this (standard) calculation for days supply means that you will be out of medication on 'day thirty'.

How a pharmacist (or your insurance company for that matter) counts a 'day supply' for a prescription can be somewhat confusing.

Complicating matters, there is no state law that provides guidance on whether or not to count the day you pick up the prescription or to start on the next day.

Additionally, in my experience at least, many pharmacists do not do a good job of explaining to patients how exactly they are counting your days.

I am a pharmacist in New York, and the controlled substance law (Part 80) only specifies that a controlled substance prescription cannot be filled more than 7 days early.

Per Part 80 (New York Controlled Substance Law):

"No additional prescriptions for a controlled substance may be issued by a practitioner to an ultimate user within 30 days of the date of any prescription previously issued unless and until the ultimate user has exhausted all but a seven days' supply of that controlled substance provided by any previously issued prescription."
Part 80 - Rules And Regulations On Controlled Substances

Regardless of what the laws are in your state, many pharmacies have their own, stricter, policies regarding how many days early you are allowed to fill your controlled substance prescription, which is what is happening in your situation.

How Do Pharmacists Count Day Supply?

In terms of how pharmacists count the 'day supply' of your prescription, standard practice is that the first day you pick up your prescription is not counted, although this is somewhat misleading because the day you finish on is not the day you are due. You are 'due' on the day you are out of medication and need to take a dose. 

This is a very key point.

The day you are 'due' for a refill, you will be out of medication.

To put it another way, you are not due for a refill on the day you run out of medication, you are due the day after (which would be the day you would need your next dose).

Let's use a 10-day antibiotic prescription example to illustrate this.

Day Supply: Example One

You pick up a 10-day antibiotic course on February 19, 2019. You want to know when you would finish the prescription, and when you would be due for a refill, (if you needed one).

Assuming you started on February 19, 2019, you would run out of your 10-day course on February 28, 2019. However, you technically are due for a refill on March 1st, 2019. This is how most pharmacists and nearly all insurance companies calculate day supply.

counting a 10 day supply calendar

Day Supply: Example Two

Now, let's use your fentanyl example.

You picked up a 30-day supply of fentanyl on February 19, 2019.

Based on how your pharmacist (and insurance company) will calculate the day supply, you will be due for a refill on March 21st, which is 30 days. Per the policy at your pharmacy, 2 days early would, therefore, be March 19th.

Remember, since you started using patches on February 19, 2019, you will finish your last dose on March 20th. However, as stated, you are due on March 21st.

Counting a 30 day supply calendar

This method of counting day supply is fairly standard, but whether or not it is in the best interest of the patient is certainly questionable.

A conversation for another article is how fair it is to hold a patient, who may have other obligations and can't make it to a pharmacy, to a pick-up date on which they will be out of medication.

Final Example

The final example I can give here is if someone was given a 'one day' supply of medication.

You wouldn't pick up and prescription and immediately be 'due' for your next fill.

You would take your single dose on the day you picked it up, and be 'due' the next day, when you had no medication left.

This is what I'm trying to illustrate in the examples above.

Final Words

Not counting the first day you pick up a prescription in your 'day supply' count isn't technically wrong, but is certainly restrictive.

Instead of thinking of your prescription being counted in terms of the day, think about it in terms of 24-hour intervals, and it makes a little more sense.

Going back to your example, you pick-up a prescription on February 19th. Let's say, at 1 PM.

Twenty-four hours (one day) from February 19th, at 1 PM, is February 20th at 1 PM. That is why the first day "isn't counted", but it technically's just that an elapsed time of 'one day' goes into the next. 

Thanks for your question and I hope this helps!

About the Pharmacist

Dr. Brian Staiger, PharmD

Dr. Brian has been practicing pharmacy for over 11 years and has wide-ranging experiences in many different areas of the profession. From retail, clinical and administrative responsibilities, he's your knowledgeable and go-to source for all your pharmacy and medication-related questions! Feel free to send him an email at!

Recent Questions