Free Opioid Conversion Calculator

Opioid Conversion: Why It Matters

Changing a patient’s opioid regimen — either switching to a different drug or altering the route of administration — is a common clinical decision when pain control is inadequate or side effects become intolerable. Because each opioid varies in potency and bioavailability, direct dose substitution can lead to under‑ or overdosing. An opioid conversion calculator relies on established equianalgesic tables to compute the morphine equivalent dose, providing a safer starting point for therapy.

Opioids work by binding to mu ($\mu$), kappa ($\kappa$), and delta ($\delta$) receptors, but their affinity and intrinsic activity differ. The route (intravenous, oral, subcutaneous, transdermal) further changes the fraction of drug that reaches the systemic circulation, which is why a single conversion chart must consider both the drug and the method of delivery. Although the opioid converter automates arithmetic, the clinician remains responsible for final dose decisions, especially in frail or complex patients.

The Equianalgesic Chart at a Glance

The relative potencies of opioids have been quantified in multiple clinical studies and are summarised in an equianalgesic chart. The table below (adapted from widely accepted sources) lists equivalent doses relative to 10 mg of intravenous morphine — the standard reference. The “relative potency” column indicates how many times more (or less) potent a drug is compared with that morphine dose.

Drug (route)	Equivalent dose (vs. 10 mg IV morphine)	Relative potency	Half‑life
Morphine (IV)	10 mg	1	2–3 h
Morphine (oral)	30 mg	$\dfrac{1}{3}$	2–4 h
Morphine (SC)	15 mg	$\dfrac{2}{3}$	n/a
Diamorphine (SC, heroin)	10 mg	1	2–3 min
Oxycodone (oral)	15 mg	$\dfrac{2}{3}$	3–4.5 h
Oxycodone (SC)	10 mg	1	3–4.5 h
Alfentanyl (SC)	1 mg	10	1.5–2 h
Fentanyl (SC)	0.200 mg	50	n/a
Hydromorphone (oral)	4 mg	$\dfrac{2}{5}$	2–3 h
Hydromorphone (SC)	2 mg	$\dfrac{1}{5}$	2–3 h
Codeine (oral)	300 mg	$\dfrac{1}{30}$	2.5–3 h
Tramadol (oral)	300 mg	$\dfrac{1}{30}$	6–8.8 h
Fentanyl (transdermal)	0.300 mg	20–27	7 h
Buprenorphine (transdermal)	0.400 mg	25	20–70 h

Note: “SC” stands for subcutaneous, “IV” for intravenous. Half‑life values are approximate and can vary with individual patient factors.

The equianalgesic chart forms the backbone of the opioid converter — it allows the tool to instantly estimate the dose that would produce equivalent pain relief for any pair of drugs and routes.

Practical Conversion: Changing the Route

Consider a patient receiving 20 mg of morphine intravenously each day. According to the relative potencies:

• Oral morphine: The oral dose needed to match 10 mg IV is 30 mg (potency $\frac{1}{3}$). Therefore, for 20 mg IV, the oral equivalent is:

  $$\text{Oral dose} = 20\ \text{mg} \times \frac{30\ \text{mg}}{10\ \text{mg}} = 60\ \text{mg/day}$$

• Subcutaneous morphine: The SC equivalent for 10 mg IV is 15 mg (potency $\frac{2}{3}$, i.e., the dose is multiplied by 1.5). Hence:

  $$\text{SC dose} = 20\ \text{mg} \times \frac{15\ \text{mg}}{10\ \text{mg}} = 30\ \text{mg/day}$$

This example illustrates why a direct 1:1 substitution between routes is never appropriate. The opioid conversion calculator automates these calculations, reducing the risk of arithmetic errors.

Accounting for Incomplete Cross‑Tolerance

When a patient has been on a long‑term opioid, tolerance develops not only to the original drug but also, to a lesser degree, to other opioids. This phenomenon is called incomplete cross‑tolerance. In practice, it means that after switching to a new opioid, the calculated equianalgesic dose should be reduced by 25–50 % to account for the fact that the patient is not fully tolerant to the new agent.

The calculator allows you to input a cross‑tolerance reduction factor. If, for example, a 30 % reduction is chosen, the final starting dose becomes:

$$\text{Starting dose} = \text{calculated equivalent} \times (1 - 0.30)$$

This adjustment improves safety and helps avoid adverse events during the transition period.

Important Safety Notes

• Always verify the calculated dose against clinical guidelines and the patient’s history.
• This opioid converter is a reference tool; it does not replace clinical judgment or individualised assessment.
• Patients should never change their opioid regimen without consulting a healthcare provider.

The tables and formulas presented here are meant to support clinicians in making informed decisions. For a full interactive experience, use the dedicated Opioid Conversion Calculator online, which already embeds the equianalgesic chart and the cross‑tolerance adjustment.