The Latin Roots of Medical Abbreviations
Medical terminology is steeped in Latin, a practice that dates back centuries. While many modern systems are shifting towards spelling out instructions for clarity, these abbreviations persist, especially on older prescription formats and in specific clinical settings. PO and OD are two prime examples, each derived from Latin phrases that dictate how and when a medication should be taken.
PO: Per Os – 'By Mouth' or 'Orally'
The abbreviation 'PO' comes from the Latin phrase per os, meaning 'by mouth'. It is one of the most fundamental abbreviations and directs the patient to swallow the medication. This can refer to tablets, capsules, or liquid oral solutions. PO is an instruction about the route of administration, specifying that the medication should enter the body via the digestive system.
- Tablets and Capsules: The most common form of PO medication. The instructions will specify the number of tablets or capsules to take.
- Oral Suspensions: Liquid medications often formulated for children or individuals who have difficulty swallowing pills.
- Dosage Specifics: The instruction '250 mg PO bid' would translate to taking 250 milligrams by mouth, twice a day.
OD: Once Daily or Right Eye – The Critical Distinction
The abbreviation 'OD' is notably more complex because it has two very different meanings depending on the context, which is why modern practice often advises against its use. The two meanings are:
- Once Daily: When used in the context of frequency for a tablet, capsule, or general medication, OD stands for omne in die, meaning 'once daily'. For example, 'Take one tablet OD' means to take one tablet once a day.
- Right Eye: In ophthalmology (eye care), 'OD' stands for oculus dexter, which is Latin for 'right eye'. This is used for medications like eye drops and refers to which eye the medication should be administered to.
This potential for confusion is why many healthcare facilities and prescribers have adopted clearer alternatives, such as writing out 'once daily' or using the abbreviation 'q.d.' (from Latin quaque die, also meaning every day).
Comparison Table: PO vs. OD
| Feature | PO (Per Os) | OD (Omne in Die / Oculus Dexter) |
|---|---|---|
| Meaning | By mouth; orally | Once Daily (medication frequency) OR Right Eye (ophthalmic use) |
| Category | Route of Administration | Dosage Frequency or Target Location |
| Derived From | Per os (Latin) | Omne in die (Latin, once daily) or Oculus dexter (Latin, right eye) |
| Common Use | Instructions for any oral medication (pills, liquids) | Instructions for once-a-day dosing (pills, etc.) or specific instructions for eye drops |
| Risk of Misinterpretation | Low, if read in context. | High, as the meaning depends entirely on the context (medication type, specialty). |
| Modern Alternative | Spelling out 'by mouth' or 'orally'. | Spelling out 'once daily' or using 'q.d.'. |
Other Common Prescription Abbreviations
To fully understand a prescription, it helps to be familiar with a few more common abbreviations, many of which are also rooted in Latin. Here is a list of other abbreviations you might encounter:
- BID: (bis in die) - Twice a day
- TID: (ter in die) - Three times a day
- QID: (quater in die) - Four times a day
- PRN: (pro re nata) - As needed
- HS: (hora somni) - At bedtime
- AC: (ante cibum) - Before meals
- PC: (post cibum) - After meals
- STAT: (statim) - Immediately
- Rx: (recipe) - Prescription
The Importance of Clarification in Patient Safety
In the past, medical professionals and pharmacists relied on these abbreviations as a kind of shorthand. However, studies and real-world experience have shown that abbreviations can lead to misinterpretation and serious medical errors, particularly in the fast-paced hospital or pharmacy environment. For this reason, many healthcare organizations have implemented policies to reduce or eliminate the use of dangerous abbreviations.
As a patient, your role in preventing these errors is crucial. When you receive a new prescription, take the time to ask your pharmacist to explain the instructions clearly. Do not assume you know what an abbreviation means. For example, if you are given an eye drop prescription and see 'OD', confirm that the pharmacist understands it means 'right eye' rather than 'once daily'. Similarly, confirm that 'PO' means you should swallow the medication and not, for example, apply it topically.
The Bottom Line for Patient Empowerment
Understanding what PO and OD mean in medical terms is a great starting point for taking control of your health. PO means 'by mouth,' a route of administration, while OD can mean either 'once daily' (frequency) or 'right eye' (location). Because of this potential for ambiguity, always verify your prescription instructions with your healthcare provider or pharmacist, even if you think you know what the abbreviations mean. By being proactive and asking questions, you can help ensure you take your medication safely and effectively. For more information on decoding your prescription, visit this comprehensive guide from Drugs.com.
