Confused about dosage and amount

So im looking to start taking Tirz and my provider is telling me the starting dose is 2.5mg (strength of the medication) and it comes with 0.5ml inside each vial which is 50 units or 5mg if im not mistaken. That being said if im just starting out if im going to stack it with Reta should i take the entire vial as instructed which is 5mg or should i take less? Also is 2.5mg strength enough to suppress my hunger? If so how long will the 2.5mg strength work for or will i get used to it and need a stronger strength of it in the future im asking because this medicine is expensive and im hoping the 2.5 mg strength will be enough.