Why this drug is interesting
Dantrolene is the textbook example of a drug whose reconstitution was the clinical problem. The old UK product, Dantrium IV, came as 20 mg vials, each needing 60 mL of sterile water and vigorous shaking, with 8–10 vials required for an average adult. Every malignant hyperthermia course in the country taught the same thing: the first job is to get four people mixing.
That product is listed as discontinued on the emc. The current UK intravenous product is Agilus 120 mg, reconstituted with 20 mL of water per vial, giving 22.6 mL of solution. Two vials cover most adults where ten used to.
Pharmacology
Mechanism
Malignant hyperthermia is a pharmacogenetic disorder of the skeletal muscle ryanodine receptor (RyR1). Triggering agents — the volatile anaesthetics and suxamethonium — cause uncontrolled calcium release from the sarcoplasmic reticulum. The result is sustained actin–myosin cross-bridging: rigidity, heat production, uncontrolled aerobic then anaerobic metabolism, CO₂ generation, acidosis, rhabdomyolysis and hyperkalaemia.
Dantrolene acts directly on the RyR1 channel to reduce that calcium release. It is not a neuromuscular blocker and does not act at the neuromuscular junction — which is why non-depolarising relaxants do not treat malignant hyperthermia and dantrolene does not abolish the need for ventilation.
Why it is not a substitute for everything else
Both labels open the warnings section with the same sentence: "The use of [dantrolene] in the management of malignant hyperthermic crisis is not a substitute for other supportive measures. These must be individually continued in their various forms."12 Stopping the trigger, hyperventilating with 100% oxygen, active cooling, treating hyperkalaemia and arrhythmia, and managing rhabdomyolysis all continue alongside it.
Dosing — Agilus (current UK product)
Initial dose
Start as soon as a malignant hyperthermia crisis is suspected. Give 2.5 mg/kg body weight by rapid intravenous injection, in adults and children alike — no paediatric adjustment.1
Repeat dosing
While tachycardia, hypoventilation, sustained hyperacidity (monitor pH and pCO₂) and hyperthermia persist, repeat 2.5 mg/kg every 10 minutes.1
If a cumulative dose of 10 mg/kg or above is being considered, re-examine the diagnosis of malignant hyperthermia.1
Vials to prepare
| Body weight | Vials to prepare | Example: dose and volume |
|---|---|---|
| Up to 48 kg | 1 | 12 kg → 30 mg → 5.6 mL · 48 kg → 120 mg → 22.6 mL |
| 49–96 kg | 2 | 72 kg → 180 mg → 33.9 mL · 96 kg → 240 mg → 45.2 mL |
| 97 kg and above | 3 | 120 kg → 300 mg → 56.5 mL · 144 kg → 300 mg → 56.5 mL |
Reconstitution
Add 20 mL water for injections to each vial and shake until dissolved. The reconstituted solution is yellow-orange with a final volume of 22.6 mL.1 Intravenous use only.
Recrudescence
The label is explicit that hypermetabolic features recur within the first 24 hours after initial resolution. If recrudescence occurs, re-administer 2.5 mg/kg every 10 minutes until the signs regress again.1 Nobody with treated malignant hyperthermia leaves critical care that day.
The discontinued product, and why it still matters
Dantrium IV 20 mg is listed as discontinued on the emc, but its regimen is the one embedded in older courses, wall charts and memories. The differences are material:
| Agilus 120 mg (current) | Dantrium IV 20 mg (discontinued) | |
|---|---|---|
| Vial strength | 120 mg | 20 mg |
| Diluent per vial | 20 mL water for injections | 60 mL sterile water |
| Volume per vial | 22.6 mL | ~60 mL |
| Vials for an initial adult dose | 2 | 8–10 |
| Filter | Not required | Single-use filter required at all times |
| Initial dose | 2.5 mg/kg | "at least 2.5 mg/kg" |
| Stated dose limits | 300 mg per dose, any weight | No per-dose maximum stated; 10 mg/kg/24 h usually sufficient, "may need to be exceeded in individual cases"; "safe uses up to 40 mg/kg have been described" |
| Key excipient | Hydroxypropylbetadex 3,530 mg/vial | Mannitol 3 g/vial |
Safety
Extravasation and pH
Both products have a solution pH of 9.5. Extravascular injection must be avoided as it can cause tissue necrosis; intra-arterial injection must be avoided because of the risk of vascular occlusion. Spillage on skin should be washed off with plenty of water.12 A reliable, well-secured large vein is part of the resuscitation.
Calcium channel blockers
Caution is also required where hyperkalaemia is already present or developing — muscular paralysis, ECG changes and bradycardic arrhythmias — including in renal impairment and digitalis toxicity.12
Hydroxypropylbetadex — the new excipient signal
Agilus contains 3,530 mg of hydroxypropylbetadex (a cyclodextrin) per vial, 156.2 mg/mL reconstituted, which is what allows the small diluent volume. The label carries an explicit warning:1
- Hydroxypropylbetadex has been associated with ototoxicity in animal studies, and audiometrically-identified hearing impairment has been observed in studies in other clinical settings.
- Cases occurred at exposures comparable to the higher range of recommended Agilus doses — mostly transient, slight to mild.
- For patients requiring above 10 mg/kg, the diagnosis should be re-evaluated — the same threshold as the diagnostic re-think.
- Exposure is expected to be higher in renal impairment, where the risk may be greater.
- Particular concern in those already at risk of hearing loss, e.g. recurrent or chronic ear infection.
Hepatotoxicity
Liver damage may occur with dantrolene and can be fatal. Agilus describes it as observed during longer-term, oral administration;1 Dantrium IV states only that it is dependent on dose and duration of therapy, without specifying the route.2 Neither is a reason to withhold emergency intravenous treatment.
Other
- Contraindication: hypersensitivity to dantrolene or excipients.1
- Non-depolarising blockade: dantrolene can enhance the effect of agents such as vecuronium.2 Expect prolonged weakness.
- Driving: major effect — weakness, dizziness and light-headedness, and Agilus adds that these may persist for up to 48 hours, during which patients must not drive or use machines.12
- Breastfeeding: discontinue during administration; both labels state breastfeeding can restart 60 hours after the last dose.12
Practical use in the ED
Malignant hyperthermia is an anaesthetic-room diagnosis, but emergency departments give volatile agents nowhere and suxamethonium routinely. The ED presentations to think about are the RSI patient who becomes rigid, hypercapnic and hot, and the patient transferred from theatre mid-crisis.
- Stop the trigger. Suxamethonium already given cannot be recalled; stop any volatile, use 100% oxygen at high flow and hyperventilate.
- Call for help and for the dantrolene. It is the one drug in the algorithm that nobody can improvise.
- Give 2.5 mg/kg rapidly, then every 10 minutes while features persist. Delegate reconstitution — even with Agilus, someone should be doing nothing else.
- Cool actively and treat hyperkalaemia, acidosis and arrhythmia on their own merits — but not with a calcium channel blocker.
- Send CK, potassium, blood gas, coagulation and urine for myoglobin; anticipate rhabdomyolysis and renal support.
- Re-examine the diagnosis at a cumulative 10 mg/kg, which is also the threshold above which the cyclodextrin warning applies.
- Admit to critical care and warn about the 24-hour recrudescence, and refer for MH unit testing and family screening.
Critical appraisal
Dantrolene has never been subjected to a randomised controlled trial in malignant hyperthermia and never will be. The Agilus label states plainly that "clinical efficacy and safety studies of Agilus have not been performed" — the product was licensed on a relative bioavailability study in 21 healthy volunteers against 20 mg intravenous dantrolene, not on clinical outcomes.1
The efficacy evidence is a retrospective review of case studies with adequate data from 1979 to 2020, and it is better than the drug's reputation for having none:1
| Adults (≥18 y) | Children (<1 month to 18 y) | |
|---|---|---|
| Patients | 116 | 91 |
| Survived | 112 (97%) | 87 (96%) |
| Median therapeutic dose | 2.4 mg/kg | 2–3 mg/kg across age groups |
| 2.5 mg/kg alone sufficient | 58% | 59% |
| Dose did not exceed 5 mg/kg | 87% | 89% |
| Dose did not exceed 10 mg/kg | 95% | 98% |
Two further figures from the label are worth having. Recrudescence is estimated at 10–15% of malignant hyperthermia patients, and is more likely in severe cases requiring higher initial doses.1 And on the reconstitution question: mean time to reconstitute one vial was 50 seconds for Agilus versus 90 seconds for a 20 mg dantrolene vial, with whole prepare-and-administer times of 1 min 53 s versus 3 min through an adult cannula.1 Multiply the second figure by the eight to ten vials the old product needed, and the case for the reformulation is made — the label notes that in published case series, quicker administration correlates with improved outcomes.
The weak point is that a well-known, well-drilled emergency regimen has been quietly replaced. Teaching materials, cognitive aids and departmental protocols written around "ten vials, 60 mL each" now describe a product that is no longer supplied, and the new label introduces a per-dose ceiling and a fresh excipient warning that the old one did not have. This is precisely the kind of change that survives in an ED for years after the box on the shelf has changed.
References
- 1Agilus 120 mg powder for solution for injection (dantrolene sodium) — Summary of Product Characteristics, Norgine Limited. electronic Medicines Compendium, product 15912. Sections 4.1–4.9, 5.1–5.3, 6.1, 6.6. Source of the 2.5 mg/kg dose, the 10-minute repeat interval, the 300 mg per-dose cap and vial table, the 20 mL reconstitution, the 24-hour recrudescence statement, the hydroxypropylbetadex ototoxicity warning, and the §5.1 case-series survival figures, recrudescence rate and reconstitution timings. Verified 31 Aug 2026.
- 2Dantrium IV 20 mg powder for solution for injection — Summary of Product Characteristics, Norgine Limited. electronic Medicines Compendium, product 1097. Listed as discontinued. Source of the 60 mL reconstitution, the filter requirement, the 3 g mannitol content, the 10 mg/kg per 24 h and 40 mg/kg statements, the calcium-channel-blocker heart-failure reports and the 60-hour breastfeeding interval. Verified 31 Aug 2026.