Hijamah means cupping. This is done to draw toxic blood from the body to reduce the intensity of the blood in the body. Several Hadiths prove it. The Holy Prophet Muhammad (may peace and blessings of Allah be upon him) got himself cupped and declared it best remedy.
خير ما تداويتم به الحجامة .
Hijamah has been declared to be mustahab in the famous Fatawa book of Hanafi Madhhab i.e. Fatawa Hindiya with reference to Zahiriyah.
و تستحب الحجامة لكل واحد كذا فى الظهيرية .
So, there is no reason being it unlawful. However, the physicians say that Hijamah is comparatively more useful for those who stay in hot areas and are of hot-natured.
Allah knows Best!
Darul Uloom Deoband