Before anyone "splits" a drug dosage, a bit of consultation with the M.D. is in order. Drug effects aren't always linear. Sometimes there are windows above or below which the drug isn't effective. Mirtazapine has an odd side-effect profile: low doses (15mg or split) tend to be very sedating whereas high doses tend sometimes to be over-stimulating, or over-energizing. Of course there may be individual differences, but your MD should be able to answer your questions. I'd hate to think what the effect of taking, for instance, sub-clinical dosages of an antibiotic. Or insulin.