In palliative care, pain management is often hampered by episodes of breakthrough pain, characterised by a rapid onset and, on average, duration less than 1 h. Until recently, only immediate release morphine and oxycodon preparations were available for the treatment of these episodes but time until effect is too long for both of these drugs. Recently, immediate release fentanyl products have become available for the treatment of breakthrough pain. These products can be classified as oromucosal and nasal products. Both are absorbed rapidly by the mucosa, although the oromucosally delivered products are partly absorbed by the gastrointestinal tract and therefore reach maximum plasma levels somewhat slower (after 30-90 min) than the nasally delivered products (after similar to 15 min). In clinical placebo controlled studies, all new immediate release fentanyl products were proven to be effective from 15 min after administration in the treatment of breakthrough pain. The first studies comparing immediate release fentanyl with immediate release morphine or oxycodon also found superiority for the new fentanyl products.