Phenobarbital is a barbiturate medication, which assists in controlling most types of childhood seizures by potentiating inhibitory neurotransmission in the brain to reduce seizure propagation. It has been available for many years as an effective treatment but its use is limited by a large number of drug interactions and the development of adverse effects (López-Muñoz, 2005).
QUESTION 1
For treating epilepsy of all forms, except typical absences, the recommended oral dose for children under 11 years old is initially 1-1.5mg/kg twice daily (Joint Formularies Committee, 2019). The licensed product available in the UK is a 15mg/5mL oral solution. What range of volumes could be administered to a 5kg child to provide doses in the appropriate starting range?
As a schedule three controlled drug phenobarbital prescriptions legally require a statement in words and figures to specify unambiguously the quantity to be supplied (Pharmaceutical Services Negotiating Committee n.d.) and are only valid for a maximum of 28 days.
QUESTION 2
What volume should be specified on a prescription for our 5kg patient for the first 28 days of treatment if they are started on 1.5mg/kg per dose?
One of the additional issues with the use of phenobarbital is that it has relatively poor solubility in water such that it produces a saturated solution at around 1mg/ml (O'Neil, 2006). However, it is much more soluble in ethanol so this is added as a co-solvent to increase the concentration of products and reduce the volumes that must be given (eMC, 2016).
QUESTION 3
Given that Phenobarbital Elixir BP contains ethanol 38%v/v (eMC 2016) how much ethanol will our 5kg patient be consuming each day from the higher starting dose of phenobarbital calculated in 1) above?
In the UK, alcohol exposure is usually considered in terms of units with a unit being equivalent to 10ml of pure ethanol (NHS, 2018).
QUESTION 4
Assuming our patient is commenced on the higher starting dose, scale up their alcohol exposure to estimate an equivalent weekly dose in terms of glasses of wine for a 70kg adult. A standard glass of 12% wine is around 2 units of alcohol.
Because of this relatively high exposure of ethanol it should be taken into account when considering medication interactions and adverse effects relating to phenobarbital.