Journal of Prescribing Practice - Drug breakdown: Penthrox

Abstract

In this column, Sharon Rees aims to refresh knowledge and interest in some of the commonly used drugs in a series of tweets. This month she is talking about #penthrox

Dr Sharon Rees @reesprescribe

Day 1: Methoxyflurane was used in the 1960s as an inhaled general anaesthetic, but was removed because of nephrotoxicity (dose related). It was repackaged (low dose) in Europe/UK circa 2020 as #Penthrox for acute moderate-severe pain relief in adult trauma and short medical procedures, such as dressing change. #Penthrox is also known as the ‘green whistle’.

Dr Sharon Rees @reesprescribe

Day 2: A #Penthrox bottle contains a single dose; conscious pts can self-administer using normal breaths. Rapid onset pain relief after ~6-10 inhalations. Effect duration ~1 hour. A second dose may be given at maximum, but the safe interval between doses is unknown. The maximum total dose in 1 week = 15 ml.

Dr Sharon Rees @reesprescribe

Day 2 (cont): In 2022 #Penthrox cost £17.89 per bottle. Advantages include pain relief without sedation, which can speed up interventions and reduce bed occupancy. Also, it is not a controlled drug. #Penthrox cannot be used to treat head injury, or where there is cardiovascular or respiratory instability

Dr Sharon Rees @reesprescribe

Day 3: #Penthrox is inhaled as a vapour. Methoxyflurane diffuses rapidly from the lungs and into the circulation. The drug is highly lipophilic and has a high volume of distribution. Liver metabolism by CYP2E1/2A6 into more soluble substances eg fluorine. Renal excretion ~ 60%, with 40% exhaled unaltered, or as CO2. The half-life is unknown.

Dr Sharon Rees @reesprescribe

Day 3 (cont): High lipophilicity and volume of distribution for #Penthrox means the drug deposits in tissue reservoirs, causing a slow release over several days; Adverse drug effects (ADEs) CAN occur hours/days post-administration. Caution re-overdose, as well as in elderly and renal/liver disease.

Dr Sharon Rees @reesprescribe

Day 4: #Penthrox mechanism of action is largely unknown; the drug reduces CNS responsivity to pain with putative interactions with GABA, glutamate and glycine receptors. Monitor post-administration for ADEs eg drowsiness; it is uncertain if it leads to muscle relaxation at low doses, but monitor as precaution.

Dr Sharon Rees @reesprescribe

Day 5: Common ADEs include dizziness, headaches, dry mouth. Serious/rare, liver injury, visual disorders and mood changes. Nephrotoxicity if a high dose is given because of accumulation of breakdown products eg fluoride ions. Higher risk if the person on drug enzyme inducers (including alcohol), or if they are a genetically fast metaboliser (NOT exhaustive).

Dr Sharon Rees @reesprescribe

Day 6: #Penthrox. There are additive drug-drug interactions (DDIs) with central nervous system depressants or for drugs with nephrotoxic effects are important. Severe DDIs with isoniazid and rifampicin re increased risk of renal toxicity. Also, DDI with methylphenidate (high risk of HTN and arrythmia) (NOT exhaustive).

Dr Sharon Rees @reesprescribe

Day 7: Abuse risk for #Penthrox is unknown, but class effects include euphoria, sedation and amnesia.

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With the help of Prescribing and Therapeutics Training Ltd and the Journal of Prescribing Practice, you can earn your 1-hour CPD certificate

In addition to the tweets, read the BNF section on anaesthesia (general), as well as the monograph on methoxyflurane. Another useful source is the summary of product characteristics for Penthrox. See the links below. Complete the additional reading suggested and then answer the ten questions. Please submit the answers to reesprescribe@gmail.com as a numbered list with TRUE/FALSE or the correct A, B, C, D option. If you achieve 8 or more out of ten on the questions, a CPD certificate will be emailed to you.

Drug breakdown: Penthrox

Abstract

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Further reading on #Penthrox