Preventive medication is not necessary for every migraine sufferer. Usually preventive medications are not considered unless a person is having at least 3 migraine attacks a month.

 

There are however special circumstances where they may be considered even if migraine attacks are not that frequent.  If a patient cannot take effective symptomatic medications because they are contraindicated, then preventive medications are especially useful.  An example would be a migraine patient with heart problems who cannot take the triptans.  Also, if migraine attacks are especially prolonged or severe, preventive medication may need to be considered even though the attacks are not all that frequent.

There are two main reasons for using preventive migraine medication.  One reason is if you are suffering significant disability from your migraine attacks even though you are using good symptomatic medication to treat your individual headache attacks.  For example, it may be that you are missing work or family activities even though you are taking medications when you experience a migraine attack.

The symptomatic migraine medications, even the triptans, do not work well for everyone.  In that situation, it is a good idea to try a prophylactic medication as well.  If it reduces the number of migraine attacks you suffer, this will help reduce your overall disability from the migraine.

Another situation where preventive medications are useful is where a person is taking symptomatic medications ;painkillers, triptans, etc. so often that he or she is at risk of developing medication induced headache.  Even though the medication may still be working well to control individual migraine attacks, over time the medication overuse can lead to more and more frequent headaches.

Other parts of this website explain this problem in more detail.  Prophylactic medications, by making the migraine attacks less frequent, will reduce the number of days a month during which symptomatic medication will be taken.

The risk of developing medication induced worsening of migraine will therefore become less, as the prophylactic mediations themselves do not produce medication overuse headache.
In general, prophylactic migraine medications are not used as much as they should be.  If you feel that they might be of benefit for you because your migraine attacks are quite frequent, or because you are taking medication frequently to treat individual migraine attacks, it may be time to discuss with your doctor whether preventive medications are for you.