My neurologist advise use Botox and odored 150 units Please advice if would be Ok use just 100 units.

neurologists usually inject the Botox based on FDA protocol to prevention headache , based on this protocol they use 155 unites in 30+ sites. Some groups use trigger point specific technique with few unites of Botox. 

Botox for migraines has a specific protocol and areas that need to be injected in order to get constant improvement on the headaches. Talk to your physician injector.Hope this helps,Dr. Gus Diaz

The number of units needed varies from patient to patient. Many physicians use less than 150 units with good results.  You can try the lower dose and if it is not enough, you can return for more.