4.4 Prescriptions, Abbreviations, Over-the-Counter & Recreational Drugs
Key Takeaways
A prescription contains the heading, the superscription (Rx), the inscription (drug, strength and form), the subscription (quantity to dispense) and the signa (patient directions).
'Sig: 1 cap p.o. t.i.d. × 7 days' means one capsule by mouth three times a day for seven days, so 21 capsules are dispensed.
Every prescription drug, over-the-counter product, natural health product and recreational drug is recorded with its dose and frequency, because these reveal bleeding, interaction and disease-control risks.
Epinephrine-containing local anesthetic is avoided within about 24 hours of cocaine or methamphetamine use, and elective care is deferred.
ISMP Canada's 'Do Not Use' list rejects U, IU, QD, OD, cc and trailing zeros, and requires a leading zero before a decimal dose.
4.4 Prescriptions, Abbreviations, Over-the-Counter & Recreational Drugs
Quick Answer: Competencies 2.6.1, 2.6.2 and 2.6.4 ask you to explain why every medication matters (prescription, over-the-counter, natural health products and illegal or recreational drugs), why dose and frequency must be recorded, and how to read a prescription. A prescription has a heading (prescriber and patient), the superscription (Rx), the inscription (drug, strength, form), the subscription (quantity to dispense) and the signa (directions, "Sig"). Know the common abbreviations: b.i.d. twice a day, t.i.d. three times, q.i.d. four times, q4h every 4 hours, p.r.n. as needed, p.o. by mouth, h.s. at bedtime, a.c./p.c. before/after meals and stat immediately. Cocaine or methamphetamine use within about 24 hours is a reason to avoid vasoconstrictor-containing local anesthetic and to defer elective care.
1. Why Every Medication, Dose and Frequency Is Recorded
The medication list is one of the most safety-critical parts of the health history (competency 2.6.2). For each drug, record:
- the name (generic and brand)
- strength and dose
- how often it is taken, and the time of the last dose
- the reason for taking it
- the prescriber.
| What the medication tells you | Examples | Effect on dental care |
|---|---|---|
| Bleeding risk | Warfarin, DOACs (apixaban, rivaroxaban), clopidogrel, daily ASA, NSAIDs, ginkgo, garlic, fish oil | Check INR or timing as directed; have local hemostatics ready; never stop a drug without the prescriber |
| Drug interactions with local anesthetic vasoconstrictors | Non-selective beta-blockers (propranolol), tricyclic antidepressants, cocaine, methamphetamine | Limit or avoid epinephrine; monitor blood pressure |
| Xerostomia and caries risk | Antidepressants, antihistamines, antihypertensives, antipsychotics, opioids, cannabis | High-fluoride regimen, saliva substitutes, frequent recall |
| Gingival enlargement | Phenytoin, calcium channel blockers (nifedipine, amlodipine), cyclosporine | Plaque control, possible gingivectomy |
| Bone healing and MRONJ | Bisphosphonates, denosumab, antiangiogenic cancer drugs | Avoid elective extractions; dental clearance before therapy |
| Disease control | Insulin and oral hypoglycemics (timing with meals), inhaled corticosteroids, antiepileptics | Schedule around meals; have the patient's inhaler or glucose source available; ask about recent changes |
| Allergies and premedication | Penicillin allergy, sulfite allergy; antibiotic premedication ordered for a high-risk cardiac condition | Choose alternatives; confirm the premedication was taken (see 4.3) |
Dose and frequency matter because they show how serious or well-controlled a condition is. A patient whose blood-pressure medication was just doubled, or who uses a rescue inhaler several times a day, carries more risk than the diagnosis alone suggests.
2. Over-the-Counter Drugs and Natural Health Products
Patients often forget non-prescription products unless asked directly (competency 2.6.1).
- ASA and NSAIDs (ibuprofen, naproxen) increase bleeding. Daily low-dose ASA is usually continued for its cardiac benefit.
- Antihistamines and decongestants cause dry mouth. Decongestants such as pseudoephedrine raise heart rate and blood pressure.
- Acetaminophen is safe for most patients, but heavy use or chronic alcohol use raises the risk of liver injury.
- Sugar-containing products (cough lozenges, syrups, chewable vitamins) used often are a hidden caries risk.
- Natural health products (in Canada, licensed products carry an NPN number) such as St. John's wort, ginkgo, ginseng and garlic interact with sedatives or affect bleeding.
3. Illegal and Recreational Drugs
Ask in a calm, non-judgmental way and explain that the question is about safety, not law enforcement. Record answers confidentially.
| Substance | Oral and systemic effects | Implications for dental treatment |
|---|---|---|
| Cannabis (legal for adults in Canada since 2018) | Dry mouth, increased caries and periodontal risk, gingival enlargement and inflammation with smoking, tachycardia, anxiety or paranoia when intoxicated | Defer elective care if the patient is impaired, because valid consent and cooperation require capacity. Expect higher anxiety and possibly reduced anesthetic effect |
| Cocaine | Intense vasoconstriction, tachycardia and hypertension; gingival ulceration where rubbed on gums; palatal perforation from snorting | Do not give epinephrine-containing anesthetic within about 24 hours of use (risk of arrhythmia, stroke, MI); defer elective care |
| Methamphetamine | "Meth mouth": rampant smooth-surface and cervical caries, severe xerostomia, bruxism, heavy sugar intake | Same vasoconstrictor caution as cocaine; aggressive prevention; expect anxiety and poor appointment attendance |
| Opioids (prescribed or illicit) | Respiratory depression, constipation, dry mouth, tolerance and dependence | Avoid adding sedatives; offer non-opioid analgesia; know the signs of overdose (pinpoint pupils, slow breathing) and the location of naloxone if the office stocks it |
| Alcohol | Oral cancer risk (much greater with tobacco), liver disease with bleeding and altered drug metabolism, dehydration | Ask about intake; caution with acetaminophen and sedatives; check for intoxication |
| Tobacco and vaping | Staining, periodontitis, delayed healing, dry socket, oral cancer | Offer brief cessation advice and resources at every visit |
Impairment at the appointment: if a patient appears intoxicated (slurred speech, odour, unsteady gait, inappropriate behaviour), inform the dentist. Elective treatment is usually rescheduled, because the patient cannot give valid informed consent and the risk of drug interactions and emergencies is higher.
4. Premedication Before Dental Treatment
Competency 2.6.3 (premedication) is covered in section 4.3. In brief:
- Antibiotic prophylaxis is limited to patients at the highest risk of infective endocarditis. Amoxicillin 2 g is given orally 30 to 60 minutes before the procedure.
- Routine prophylaxis is not recommended for prosthetic joints.
- Some patients are premedicated with an anxiolytic (for example, a benzodiazepine taken before the appointment). These patients need an escort and must not drive.
Always confirm that the prescribed premedication was taken, and record the drug, dose and time.
5. Reading a Prescription
Dental assistants do not prescribe, but they must interpret prescription information correctly (competency 2.6.4). They do this when entering it in the chart, answering a patient's question or checking that a medication history matches what was dispensed.
| Part | Contents | Example |
|---|---|---|
| Heading | Prescriber's name, address, phone and licence/registration number; patient's name, address and date of birth or age; date | Dr. A. Nguyen, DDS ... Patient: J. Tremblay, DOB 1988-04-12 |
| Superscription | The symbol Rx ("take thou") | Rx |
| Inscription | Drug name, strength and dosage form | Amoxicillin 500 mg capsules |
| Subscription | Instructions to the pharmacist, especially the quantity to dispense | Disp: 21 (twenty-one) capsules |
| Signa (transcription) | Directions for the patient, written after "Sig:" | Sig: 1 cap p.o. t.i.d. × 7 days |
| Refills, signature | Number of refills (if any) and the prescriber's signature | No refills |
The example means: take one capsule by mouth three times a day for seven days, and the pharmacist dispenses 21 capsules.
Common Abbreviations
| Abbreviation | Meaning | Abbreviation | Meaning |
|---|---|---|---|
| Rx | Prescription / take | Sig | Label / directions |
| Disp | Dispense | p.o. | By mouth |
| b.i.d. | Twice a day | t.i.d. | Three times a day |
| q.i.d. | Four times a day | q4h | Every 4 hours |
| p.r.n. | As needed | h.s. | At bedtime |
| a.c. | Before meals | p.c. | After meals |
| stat | Immediately | NPO | Nothing by mouth |
| tab / cap | Tablet / capsule | gtt | Drops |
| mg / mL | Milligram / millilitre | mcg | Microgram |
Error-prone abbreviations: ISMP Canada's "Do Not Use" list warns against several abbreviations and number formats. Do not use U or IU (misread as 0 or 10), QD or OD for "daily" (OD is also read as "right eye"), or cc (write mL). Never write a trailing zero (write "1 mg", not "1.0 mg"). Always write a leading zero ("0.5 mg", not ".5 mg"). Opioids and other controlled drugs have extra provincial requirements (for example, quantities written in words as well as numbers, and prescription monitoring programs). Prescription records are part of the patient's chart and must be legible, complete and signed.
A prescription reads: 'Ibuprofen 400 mg tabs. Disp: 20. Sig: 1 tab p.o. q.i.d. p.r.n. pain.' What do the patient directions mean?
Take one tablet by mouth once a day at bedtime when pain is present
Dissolve one tablet under the tongue four times a day until all are used
Take one tablet by mouth every four hours, whether or not there is pain
Take one tablet by mouth up to four times a day as needed for pain
Which part of a prescription gives the pharmacist the quantity to dispense?
Superscription
Subscription
Signa
Inscription
A patient admits to using cocaine about three hours before a scheduled restorative appointment. What is the most appropriate response?
Inform the dentist; elective care is deferred and epinephrine-containing anesthetic is avoided
Proceed as planned, because cocaine is fully cleared from the body within about an hour
Proceed with a reduced dose of anesthetic with epinephrine to limit the drug interaction
Begin nitrous oxide sedation first so the patient is calmer before the anesthetic is given
Following ISMP Canada's 'Do Not Use' guidance, which dose is written correctly?
5.0 mg
5 U
.5 mg
0.5 mg
Sections you finish are checked off in the contents.