Its also possible to be completely bald over the crown of your head
The half life of a B12 injection averages about 6 days from the studies I've seen so the answer is yes
Comprehending the variations in texture and development pattern of your hair may aid you in selecting suitable products, styles, and treatments that enhance your inherent hair attributes

Produce antioxidant and anti-inflammatory effects Remove damaged tissue proteins Reduce infections and heal wounds Lower nerve damage and pain Boost skin collagen production by 70% Reduce wrinkles and fine lines by 35% Tighten loose skin and reverse thinning of aged skin Repair protective skin barrier proteins Improve skin firmness, elasticity, and clarity Reduce fine lines, depth of wrinkles, and improve the structure of aged skin Smooth rough skin Reduce photodamage, mottled hyperpigmentation, skin spots, and lesions Improve overall skin appearance Stimulate wound healing Protect skin cells from UV radiation Reduce inflammation and free radical damage Increase hair growth and thickness, enlarge hair follicle size GHK-Cu may also protect the body against liver fibrosis and serve as an anti-cancer therapy
At first glance, this seems contradictory