A vitamin medicine can still raise an allergy question
The MHRA’s December 2023 alert explains that hydroxocobalamin and cyanocobalamin medicines contain cobalt. People with known cobalt allergy should discuss that history when B12 treatment is prescribed. [1]
Sensitivity reactions can involve rash or hives and can appear promptly or after a delay. A rash description alone cannot establish its cause. Report the preparation and timing to a healthcare professional. [1]
Severity determines the urgency
The MHRA advises urgent medical care for signs of a serious reaction, including extensive or blistering rash, wheeze, breathing difficulty or feeling faint. Do not conduct a deliberate repeat exposure to test a suspected allergy. [1]
The alert does not treat a history limited to skin sensitivity as an automatic prohibition on B12 treatment. A previous serious reaction requires an individual benefit–risk assessment. The prescriber should decide how to continue necessary deficiency care. [1]
Preserve the timing in the report
Imagine a rash appears the day after B12 treatment. The delay is relevant information rather than a reason to dismiss a possible reaction. Record the medicine, time given, symptom onset and other exposures for assessment.
| Finding | Interpretation |
|---|---|
| Exact B12 medicine | Identifies the exposure |
| Time to symptoms | Include delayed onset |
| Breathing difficulty or faintness | Urgent care |
Sources for this article
- MHRA: Vitamin B12 and cobalt allergy
Source consulted: 2026-09-17. This is a reference resource, not a claim that we conducted the studies or tested a product.