Module 10 of 14 · 14 min
Conversions between products & units
Switching IV to oral, one steroid or opioid to another, or reading vitamin D in IU vs mcg and iron as salt vs elemental is daily pharmacy work, and exam questions expect you to use the equivalence the question gives you.
🧠 "Total, Turn, Trim, Divide": total the day, turn with the ratio (new on top), trim for cross-tolerance, divide into doses.
📌 Memorize these
- Vitamin D
- $1\,\text{mcg} = 40\,\text{IU}$
- Prednisone : methylprednisolone
- $5\,\text{mg} : 4\,\text{mg}$
- Oral morphine : oral hydromorphone
- $30\,\text{mg} : 6\,\text{mg}$ratios used in this module
- IV hydromorphone : oral morphine
- $1.5\,\text{mg} : 30\,\text{mg}$
- Different opioid
- reduce by 25–50%the question states how much
- Same drug, new form
- no reduction
An equivalence table is a ratio: if prednisone 5 mg ≡ methylprednisolone 4 mg, then $1\ \text{mg methylprednisolone}=\tfrac{5}{4}\ \text{mg prednisone}$. Put the drug you want on top.
Always convert the total daily dose first, then divide into the new schedule (q12h, q4h…).
Salts contain less active moiety than their weight: $\text{elemental}=\text{salt mass}\times\dfrac{\text{MW of element}}{\text{MW of salt}}$ (or the stated % elemental). Vitamin D: $1\ \text{mcg}=40\ \text{IU}$.
When switching to a different opioid, reduce the calculated equianalgesic dose by about 25–50% (the question will state the amount) for incomplete cross-tolerance. Same drug, new formulation: no reduction.
🧭 The method
- 1Total the current dose per 24 h (include all routes and doses).
- 2Write the equivalence given as a fraction with the NEW drug/route on top.
- 3Multiply to get the equivalent daily dose of the new product.
- 4Apply any stated cross-tolerance reduction (different opioid only).
- 5Divide by the number of daily doses and convert to tablets or mL.
Before each reveal, predict the next step — then check.
A patient takes morphine SR 60 mg PO q12h. Switch to oral hydromorphone q4h. Use: oral morphine 30 mg ≡ oral hydromorphone 6 mg, and reduce by 25% for incomplete cross-tolerance. What is each hydromorphone dose?
- 1
Total daily morphine.
$$60\ \text{mg}\times2=120\ \text{mg morphine/day}$$ Convert with the ratio (hydromorphone on top).
$$120\ \text{mg morphine}\times\frac{6\ \text{mg hydromorphone}}{30\ \text{mg morphine}}=24\ \text{mg hydromorphone/day}$$Different opioid: reduce by 25% (keep 75%).
$$24\times0.75=18\ \text{mg/day}$$Divide into q4h dosing (6 doses per day).
$$\frac{18\ \text{mg/day}}{6\ \text{doses/day}}=3\ \text{mg per dose}$$
Same method — the last steps are yours. Work on paper, then reveal.
A patient takes methylprednisolone 12 mg BID and is switched to once-daily prednisone. Use methylprednisolone 4 mg ≡ prednisone 5 mg. What daily prednisone dose is equivalent?
- 1
Total daily methylprednisolone.
$$12\ \text{mg}\times2=24\ \text{mg/day}$$ - 2
Your turn
Now you: convert with prednisone on top.
Work it out on paper, then reveal.
$$24\ \text{mg}\times\frac{5\ \text{mg prednisone}}{4\ \text{mg methylpred}}=30\ \text{mg prednisone/day}$$
Type your number. Your first try decides when this module comes back for review.
A vitamin D3 capsule contains 50 000 IU. How many mcg is that? (1 mcg = 40 IU)
Divide IU by 40.
= 1250 mcg
A patient on prednisone 50 mg daily is changed to dexamethasone. Use dexamethasone 0.75 mg ≡ prednisone 5 mg. What daily dexamethasone dose is equivalent?
Dexamethasone goes on top of the ratio.
= 7.5 mg
How many mg of elemental iron are in one ferrous sulfate heptahydrate 325 mg tablet? (MW FeSO₄·7H₂O = 278; Fe = 55.85)
Multiply the salt mass by the fraction of the molecule that is iron.
= 65.3 mg
A 64 kg patient on IV co-trimoxazole (trimethoprim component 15 mg/kg/day, given q8h) is switched to oral DS tablets (trimethoprim 160 mg each). Oral bioavailability is essentially complete, so use the same dose. How many DS tablets per dose?
Calculate trimethoprim per day, divide by 3 doses, then by 160 mg.
= 2 tablets
A patient on morphine SR 60 mg PO q12h is switched to immediate-release morphine oral solution 5 mg/mL q4h (same drug, same route). How many mL per dose?
Same drug and route: no cross-tolerance reduction. Total daily, then 6 doses.
= 4 mL
A patient receives hydromorphone 0.6 mg/h by continuous IV infusion. Convert to oral morphine using IV hydromorphone 1.5 mg ≡ oral morphine 30 mg, then reduce by 30% for incomplete cross-tolerance. What is the new total daily oral morphine dose (mg)?
Hourly × 24 first. Then the ratio, then keep 70%.
= 201.6 mg/day
⚠️ Tricky
A patient stable on IV phenytoin sodium 300 mg daily is switched to oral phenytoin suspension 125 mg/5 mL, which contains phenytoin free acid. Phenytoin sodium is 92% phenytoin (S = 0.92). To deliver the same amount of phenytoin, how many mL per day?
Convert the salt to the amount of phenytoin itself before converting to suspension volume.
= 11.04 mL
⚠️ The trap: Converting mg for mg (ignoring the salt factor) gives 12 mL, about 8% too much phenytoin.
⚠️ Tricky
Calcium chloride 10% injection contains the dihydrate CaCl₂·2H₂O (MW 147; anhydrous CaCl₂ MW 111). How many mEq of calcium are in one 10 mL ampoule? (Ca²⁺ valence = 2)
10% of 10 mL is 1 g of the dihydrate; use the dihydrate MW, then multiply mmol by the valence.
= 13.6 mEq
⚠️ The trap: Forgetting the valence gives 6.8 (mmol, not mEq), and using the anhydrous MW of 111 gives 18.0 mEq.
⚠️ Tricky
A patient is ordered 15 mmol of phosphate as potassium phosphate injection (each mL contains 3 mmol phosphate and 4.4 mEq potassium). How many mEq of potassium will the dose also deliver?
The order is in phosphate; find the volume first, then the potassium in that volume.
= 22 mEq
⚠️ The trap: Assuming mmol of phosphate equals mEq of potassium gives 15 mEq and under-counts the potassium load by 7 mEq.
⚠️ Tricky
A patient takes oxycodone CR 40 mg q12h and on average three 10 mg oxycodone IR breakthrough doses daily. Switch to oral hydromorphone q4h using oral oxycodone 20 mg ≡ oral hydromorphone 4 mg, then reduce by 25% for incomplete cross-tolerance. What is each q4h hydromorphone dose (mg)?
The 24-hour total includes the breakthrough doses actually used.
= 2.75 mg
⚠️ The trap: Leaving out the 30 mg/day of breakthrough oxycodone gives 2 mg q4h, under-treating the pain.
⚠️ Tricky
A child is to receive vitamin D3 800 IU daily. The oral solution is labelled cholecalciferol 0.125 mg/mL (1 mcg = 40 IU). How many mL per day?
IU → mcg → mg, so the dose and the label are in the same unit.
= 0.16 mL
⚠️ The trap: Dividing 20 mcg by 0.125 without converting mcg to mg gives 160 mL — a thousandfold error.
🕵️ Spot the mistake
A patient on prednisone 20 mg daily needs hydrocortisone instead. Use hydrocortisone 20 mg ≡ prednisone 5 mg. What daily hydrocortisone dose?
A student wrote
⚠️ Traps examiners set
- Inverting the equivalence ratio (check: the less potent drug needs the bigger number).
- Converting a single dose instead of the 24-hour total.
- Forgetting the cross-tolerance reduction when changing opioids — or applying it when only the formulation changes.
- Reporting salt weight when the question asks for elemental iron.
- Mixing up IU and mcg for vitamin D (1 mcg = 40 IU).
💬 Explain it in your own words
Why do we reduce the dose when switching to a different opioid but not when switching morphine SR to morphine immediate-release?