What Does BIW Mean on a Prescription and Why Is it Risky?

October 7, 2025 •

4 min read

According to the Institute for Safe Medication Practices (ISMP), using ambiguous abbreviations like BIW on a prescription is a dangerous practice that can lead to medication errors. So, what does BIW mean on a prescription? It means 'twice weekly', and understanding this is critical for your health.

Quick Summary

The medical abbreviation BIW on a prescription indicates a medication should be taken twice weekly. Healthcare professionals now discourage its use due to confusion with BID (twice daily), which can cause serious dosing mistakes.

Key Points

BIW Means Twice Weekly: The medical abbreviation BIW stands for bis in septem diebus, which means twice per week.
High Risk of Confusion: BIW is often confused with BID (bis in die), which means twice daily, leading to potentially dangerous overdoses.
Disapproved Abbreviation: Major patient safety organizations, such as ISMP, recommend against using BIW and other ambiguous abbreviations to prevent medication errors.
Spell Out for Safety: The safest practice is for healthcare providers to write out the full, clear instruction, such as "twice weekly".
Patient Vigilance is Key: Patients should always ask their doctor or pharmacist for clarification on confusing instructions and verify the medication label matches their understanding.
Errors Have Occurred: Real-world examples show how misinterpreting BIW as twice daily can lead to a potentially fatal error if not caught by a pharmacist.
Use Technology for Reminders: For medications like BIW that require less frequent dosing, using a pillbox or phone reminders can help ensure you take your medication on the correct days.

Medical prescriptions often contain abbreviations derived from Latin to communicate instructions efficiently. While historically common, many of these shortcuts are now recognized as error-prone and dangerous. Among these, the abbreviation BIW, meaning "twice weekly," is a prime example of a term that can lead to serious patient harm due to potential confusion with BID, which means "twice daily". Recognizing the meaning of these abbreviations is the first step, but being aware of the risks and advocating for safer practices is crucial for patient safety in pharmacology.

What BIW Means on a Prescription

BIW is a medical abbreviation for bis in septem diebus, which is Latin for "twice in seven days" or, more simply, twice a week. This instruction specifies a less frequent dosing schedule than medications taken daily. You might see BIW prescribed for certain long-acting medications, such as some injections for autoimmune diseases or specific oral medications where a steady state is not required daily. The intended dosing frequency is much lower than for more common prescriptions, which is exactly why misinterpretation can be so harmful.

Why BIW is Considered a Dangerous Abbreviation

The Critical Confusion with BID

The primary danger of BIW lies in its close resemblance to another, more common, and frequently used abbreviation: BID.

BIW: Twice a week (e.g., every 3.5 days).
BID: Twice a day (e.g., every 12 hours).

This simple, one-letter difference has been the cause of documented medication errors. A patient or pharmacy technician might misread or misenter "BIW" as "BID" leading to a medication being administered 14 times more frequently than intended (twice daily instead of twice weekly). This kind of overdose can be extremely dangerous, depending on the medication, and can cause severe side effects or toxicity.

Official Recommendations Against Ambiguous Abbreviations

Due to documented risks, numerous organizations, including the Institute for Safe Medication Practices (ISMP) and The Joint Commission, have actively campaigned for and implemented lists of "do not use" abbreviations. These organizations recommend that healthcare professionals and electronic prescribing systems explicitly spell out the frequency rather than using potentially confusing abbreviations. The goal is to eliminate any chance of misinterpretation by using clear, plain language, such as "twice weekly".

Comparison of Common Frequency Abbreviations

To highlight the potential for confusion, here is a comparison table of frequently used (and sometimes problematic) frequency abbreviations:


Abbreviation	Latin Term	Meaning	Potential for Error	Safer Alternative
BIW	Bis in septem diebus	Twice weekly	High risk of confusion with BID, leading to a massive overdose.	Twice weekly
BID	Bis in die	Twice daily	Moderate risk of confusion with BIW and BT (bedtime).	Twice daily or Every 12 hours
TID	Ter in die	Three times daily	Moderate risk of confusion with TIW (three times weekly).	Three times daily
TIW	Ter in week	Three times weekly	High risk of confusion with TID, leading to an overdose.	Three times weekly
QD	Quaque die	Every day	High risk of confusion with QID (four times daily) and QOD (every other day).	Daily or Every day
QID	Quater in die	Four times daily	High risk of confusion with QD (every day).	Four times daily
QOD	Quaque altera die	Every other day	High risk of confusion with QD (every day).	Every other day

What Patients Can Do to Ensure Medication Safety

As a patient, you play a vital role in preventing medication errors. Here are a few essential steps you can take:

Always ask for clarification: When your doctor prescribes a new medication, don't hesitate to ask for the full, spelled-out instructions. Ask questions like, "How often do I take this?" and "Which days should I take it?"
Review your pharmacy label: When you pick up your medication, read the label carefully. The pharmacy's computer system should have already expanded the abbreviation to plain language like "Twice weekly." If you see an abbreviation, or if the instructions seem unclear, ask the pharmacist to explain them.
Keep an updated medication list: Maintain a list of all your medications, including the drug name, dosage, and frequency. This is helpful for all healthcare providers and ensures consistency in your treatment.
Report inconsistencies: If you notice a discrepancy between what your doctor told you and what the pharmacy label says, immediately contact your pharmacy or doctor. The case mentioned by the ISMP involved a vigilant pharmacist catching the error before it reached the patient.
Use visual aids: For complex medication schedules, use a pillbox or set phone reminders to help you remember. This is especially helpful for twice-weekly dosing, where it is easy to lose track of which days you have already taken your dose.

The Evolution of Prescription Writing and the Push for Clarity

The move away from error-prone abbreviations like BIW and toward clear, unambiguous language is part of a broader shift in healthcare toward improved communication and patient safety. Electronic health records (EHRs) and e-prescribing systems now often incorporate safety checks that flag or automatically expand dangerous abbreviations. While these systems are a powerful tool, they are not foolproof, and human vigilance remains the last line of defense against potential harm. The Institute for Safe Medication Practices (ISMP) provides valuable resources on this topic, promoting safe medication practices for both healthcare professionals and patients (for more information, see ISMP's website).

Conclusion

While BIW simply means "twice weekly," the potential for misinterpretation in a complex medical environment makes it a dangerous abbreviation. The risk of confusing BIW with BID and the subsequent potential for overdose is a serious patient safety concern that has prompted official bodies to recommend its abandonment. By being proactive and communicating clearly with your healthcare providers and pharmacists, you can help eliminate these risks and ensure you are taking your medication safely and correctly.

Frequently Asked Questions

BIW is a medical abbreviation that stands for "twice weekly". It indicates that a medication should be taken two times during a seven-day period.

The key difference is the frequency: BIW means twice a week, while BID means twice a day. This single-letter distinction is a major source of medication errors.

BIW is considered dangerous because it is easily mistaken for BID. This confusion can cause a massive dosing error, resulting in a patient taking a medication 14 times more often than prescribed, which could lead to an overdose.

Yes, many abbreviations have been flagged as error-prone. These include QD (once daily) which can be confused with QOD (every other day), and TID (three times daily) which can be confused with TIW (three times weekly).

If you see BIW, it's safest to ask your pharmacist to confirm the instructions and ensure the label says "twice weekly." Many pharmacies will already have expanded the abbreviation to avoid confusion.

Healthcare professionals are trained to avoid ambiguous abbreviations and, with the aid of electronic health records, are encouraged or required to spell out instructions clearly, such as writing "twice weekly" instead of BIW.

The safest way is to read the full, spelled-out instructions on the pharmacy label. Always confirm the frequency with your doctor and pharmacist, and use a pill organizer or calendar to track your weekly dosing schedule.