In a field of rare diseases such as aspergillosis there is often a serious lack of fully tested and authorised medications for use by doctors. This leads directly to the use of medications which are not authorised to treat the disease, or are not authorised for use in a particular patient group or for use in a particular way - this is referred to as off-label use. This is completely legal in many countries as long as it is carried out by doctor as phsicians have the legal power to do so - they are however doing so without the authorised guidelines and must use their own experience and judgement.
For example the current approval in the US for the use of the antifungal voriconazole is as follows:
Vfend [voriconazole] has been approved by the FDA for the treatment of deadly fungal infections. The medication is indicated for the primary treatment of acute invasive aspergillosis.The approval in the EU by the European Medicines Agency is very similar. Note that no mention is made of treating chronic aspergillosis, a purpose for which it is used every day by many patients. Likewise itraconazole is not approved for use in allergic broncho-pulmonary aspergillosis (ABPA), or severe asthma with fungal sensitivity (SAFS) for which it has proven benefit.
The central problem seems to be that formal approval is so complicated it takes a very long time to be granted, even when there are excellent cases for approval.
This situation is less than ideal and the UK NHS have decided to begin providing expert assessments for off-lable drug use via the National Institute for Clinical Excellence (NICE). The principle will be to provide a summary based on current available evidence which doesn't replace formal guidence but will support local decision making. Read more here.