It is complicated to decide DDDs for the different dosage forms and even the different inhalation devices of the same dosage form. It has been shown that certain inhalation devices give a better deposition of the active ingredient in the lungs. This gives a better clinical effect, and therefore the active ingredients can be used in lower dosages. It has been decided not to take this aspect into consideration when assigning DDDs in this group, since the picture is very complex and satisfactory comparative documentation is not always available. Accordingly, only one DDD is assigned for one dosage form of a substance (e.g. powder inhalation).
For some substances, the labelling of the strength of identical inhalation products may differ between countries. In some countries, metered dose (measured as the amount of substance released from the inhaler with the mouthpiece removed) is used while in other countries delivered dose (measured as the amount of substance released from the inhaler with the mouthpiece in place) is used in the labelling. Delivered dose will usually be lower than metered dose. This is important to take into considerations when linking DDD information to the products in the different countries.
The DDDs for inhalation aerosol and inhalation powder of the same substance are in most cases given the same DDD value. The DDDs for inhalation solutions are, however, different from these and much higher, partly because less amount of the active ingredient will reach the target organ, and partly because this dosage form often is used in more severe asthma.