can you inject b12 into your thigh How to Give Yourself Intramuscular (IM) Injections | Patients & Families Tirzepatide injection site thigh: guide
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But beware, there is a tendency for the doctor to think the more symptoms a person reports, the more likely the cause of the symptoms is psychological

Supplementation of vitamin B12 COMPOSITION Cyanocobalamin (Vitamin B12) 1mg/mL ACTIONS Supplementation of Vitamin B12 INDICATIONS Vitamin B12 supplement for the treatment of macrocytic anaemia in horses and dogs DOSAGE AND ADMINISTRATION Administer intramuscularly twice weekly or as directed by a veterinary surgeon

It doesnt last for long though, so you do need to get more often

In older adults or those with renal/hepatic impairment, individualized dosing and monitoring may be needed
