Using PRN or "As Needed" Medicines Safely

Published: March 15, 2021
Christina DeRemer
By Christina E. DeRemer, PharmD, BCPS

Some medications come with specific instructions for use every day, such as “Take 1 tablet by mouth every 8 hours.”

However, other medications are only used when needed for a specific situation, such as intermittent chest pain, the common cold, allergies, constipation, or pain. Some of these medicines are prescribed for you by your physician while others can be purchased at your local pharmacy.

Medicines that are taken “as needed” are known as “PRN” medicines. “PRN” is a Latin term that stands for “pro re nata,” which means “as the thing is needed.”

It’s important to know the difference between daily and “as needed” medicines. When you look at your list of medicines, do you know which ones are supposed to be taken every day and which ones can be taken every once in a while to treat certain symptoms?

For example, you may not feel that your blood pressure or diabetes medication is helping you every day. However, these medications are only effective if taken every day.

If you are prescribed a medicine to take “only as needed,” the pharmacist should provide you with clear instructions about how and when to take it. These instructions should include the following information:

  • How much medicine you can take in a set period of time. For example, many people take nitroglycerin tablets that melt under the tongue for chest pain. But if you use three doses in 15 minutes and symptoms don’t go away, you should seek immediate medical attention.
  • When to take your PRN medication. For example, you may have been diagnosed with heart failure and your physician has instructed you to weigh yourself daily. As-needed “water” pills should only be taken if your scale shows a 3-lb. weight gain in 24 hours.
  • When to take scheduled and PRN medications for a single health problem. For example, if you recently had surgery for chronic hip and knee pain, your doctor may have added some as-needed pain medications to your prescription medications for pain. It’s important to know what order to take these in and how long to wait between doses.

It's always a good idea to discuss any medication questions you have with your pharmacist. That’s because many as-needed medicines contain similar ingredients. For example, acetaminophen is contained as an ingredient in many prescription and non-prescription medicines. If you take several of these medicines together, these products could result in an overdose of that ingredient. It’s important that you read the label carefully.

PRN or as-needed medicines are an important part of the therapies that people take to manage different health conditions. To make sure that you take your medicines safely, always ask your pharmacist if you have a question.

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