This is taken verbatim from the JAA Manual of Civil Aviation Medicine :
Aeromedical Status
Migraine cannot be treated in pilots with ergotamine and other drugs, because such medications are
unacceptable in aviation due to side effects. Known migraineurs should not be selected for professional
aircrew training due to the unpredictability and disabling nature of the condition, but those who present
after qualification either through worsening of the condition or an in-flight incident should be neurologically
assessed and any causal factor addressed. If no other structural abnormality is found and the individual is
migraine free for 3 to 6 months, a return to flying may be approved with a multi-pilot (Class 1 ‘OML’)
limitation. A fit assessment for solo operations must be evaluated against the history and type of
operation. Some prophylactic treatments such as propranalol, may be acceptable with each case being
reviewed individually.
A similar assessment applies to Class 2.
The following must be considered:
• Frequency of headaches
• Degree of incapacitation caused by the headache.
• Drugs used to treat the headache.
Adverse factors for aeromedical certification include:
- Sudden significant neurological symptom such as loss of vision, weakness and incoordination with no
warning
- Failure or of prophylactic treatment with frequent attacks
- Requirement for intensive treatment
- Short prodrome that does not allow effective use of acute treatment before symptom onset.
Hope that's helpful, at least as a starter.