N, in the injectable form already contains around 10% alcohol. You don't need to add more to make it lethal, as long as you're already taking a high enough dose (indicated by most clinics to be more than 12 grams, to ensure a more rapid death).
However any additional amount will potentiate the effects, by prolonging the 'sleep duration', but it would also potentially increase the likelihood of vomiting.
As @Forveleth said, additional alcohol isn't required, but a shot (25ml) of a spirit is often used after drinking the N to help reduce the after taste.