SIR–Zier et al. demonstrated the efficacy and tolerability of an inhaled nitrous oxide ⁄oxygen mixture for sedation and analgesia for intramuscular botulinum toxin injections in children. We wish to highlight issues concerning the wider applicability of the results of their study to the outpatient setting not addressed in their discussion. To aid this discussion, we submit data from our recent practice with intramuscular botulinum toxin injections in a similar group of children in a day-case ward setting using Entonox (BOC Gases Ltd, Worsley, UK) or enteral midazolam (Table I). Entonox is a fixed 50% nitrous oxide ⁄oxygen mixture available as a portable gas cylinder, unlike the operatortitrated 70% maximum nitrous oxide ⁄oxygen mixture delivered by a continuous flow system in Zier et al.’s study. The latter requires specialist training and machinery that is not typically available in the outpatient setting. In the UK, Entonox is classed as a ‘pharmacy product’ rather than a ‘prescription only medicine’. This allows it to be used by non-medical practitioners who have received training in line with their employing institution’s guidelines. In our institution, the pain service trained two of the authors who are physiotherapists (KL, CS) to use Entonox for procedural pain control in the outpatient and day-case ward setting. Entonox is typically administered to the child via a demand-valve attached to a face-mask or mouthpiece. Thus administration requires the conscious cooperation of the child, unlike the system described in Zier et al.’s study. Self-administration is both a safety feature (as the child must remain conscious) and a limitation of Entonox (as the child must have appropriate cognitive and behavioural capacity to comply with instructions). By comparison, the continuous flow system is able to deliver a higher nitrous oxide concentration (70%) allowing deeper sedation and presumably analgesia, and it is less constrained by the need for compliance from the child. However, the favourable safety profile of nitrous oxide quoted in Zier et al.’s study relates to the fixed 50% nitrous oxide ⁄oxygen mixture. Serious adverse events are rare, but unconsciousness and significant desaturation have been reported with 70% nitrous oxide. Table I summarizes data from 40 consecutive procedures on 33 children (21 males; mean age 8y 1mo, [SD 4y 11mo, range 1y 11mo–17y 5mo)] predominantly with spasticity due to cerebral palsy (GMFCS levels: I=11, II=6, III=6, IV=5, V=5; hemiplegia=8, diplegia=11, quadriplegia=7, other=7) performed in our day-care ward using Entonox or midazolam with topical analgesia. In our experience, not all children are equally suitable for either treatment, e.g. midazolam use is not limited by the child’s capacity to self-administer sedation. In general, Entonox is our first choice for children over 5 years old who are likely to comply with verbal instruction.