What You Should Know About Hydroxocobalamin Side Effects

If your doctor has prescribed Hydroxocobalamin — a form of vitamin B12 given by injection — you're probably wondering what side effects to expect. The short answer: most people tolerate it well, and the most common side effects are mild. But there are a few things worth knowing, including one side effect that can be alarming if you're not prepared for it.

This guide covers the common side effects, the serious ones to watch for, and when you should call your doctor.

What Is Hydroxocobalamin?

Hydroxocobalamin is a form of vitamin B12 used to treat B12 deficiency and pernicious anemia. It's given as an intramuscular (IM) injection, typically 1,000 mcg per dose. There's also an intravenous (IV) form called Cyanokit, used in hospitals to treat cyanide poisoning.

Unlike Cyanocobalamin (the other common B12 injection), Hydroxocobalamin stays in the body longer, which means you may need fewer injections over time. To learn more about how Hydroxocobalamin works, check out our detailed guide.

Common Side Effects

These side effects are reported frequently and are usually mild. They often improve on their own as your body adjusts to the medication:

• Injection site pain, swelling, or redness — This is the most common side effect. The injection goes into a muscle (usually the upper arm or thigh), and some soreness is normal. It typically fades within a day or two.
• Red discoloration of urine — This is perhaps the most surprising side effect. Hydroxocobalamin can turn your urine red, pink, or orange. This is harmless and happens because your body is excreting the excess. It can last anywhere from a few days to 5 weeks after a dose. This is not blood in your urine — it's just the red color of the medication.
• Red discoloration of skin — Some patients notice a reddish tint to their skin, especially after higher doses (like the IV form used for cyanide poisoning). This is temporary and fades over days to weeks.
• Nausea — Some people feel mildly nauseated after their injection. Eating a small meal before your shot may help.
• Headache — Mild headaches are reported by some patients, usually resolving within a few hours.
• Diarrhea — Occasional loose stools may occur, especially in the first few days of treatment.
• Acne or skin rash — Some patients notice breakouts or mild skin irritation, which is generally temporary.

Serious Side Effects

Serious side effects from Hydroxocobalamin are rare, but they do happen. If you experience any of the following, contact your doctor right away or go to the emergency room:

• Severe allergic reaction (anaphylaxis) — Signs include hives, swelling of the face or throat, difficulty breathing, and dizziness. This is extremely rare but requires immediate medical attention.
• Dangerously high blood pressure — Hydroxocobalamin can raise blood pressure in some patients. Symptoms may include severe headache, blurred vision, chest pain, or confusion. This is more common with the high-dose IV form.
• Chest pain or irregular heartbeat — If you experience chest tightness, a fast or pounding heartbeat, or pain that spreads to your arm or jaw, seek emergency care.
• Severe shortness of breath or wheezing — Difficulty breathing after an injection could signal an allergic reaction or another serious issue.
• Blood clots (peripheral vascular thrombosis) — Though rare, some patients have experienced blood clots. Watch for sudden pain, swelling, or warmth in a leg, or unexplained shortness of breath.
• Increased sun sensitivity — While your skin has a red discoloration from the medication, you may be more sensitive to sunlight. Avoid prolonged sun exposure and wear sunscreen.

Side Effects in Specific Populations

Pregnant Women

Hydroxocobalamin is FDA Pregnancy Category C, meaning there isn't enough human data to rule out risk. For B12 deficiency during pregnancy, your doctor will weigh the benefits against potential risks. For cyanide poisoning in pregnancy, the benefits clearly outweigh the risks.

Breastfeeding

It's not known whether Hydroxocobalamin passes into breast milk. Breastfeeding is generally not recommended immediately after high-dose IV treatment. Talk to your doctor about timing.

Older Adults

Older adults may be more susceptible to blood pressure changes. Your doctor may monitor your blood pressure more closely during treatment.

People with Heart or Kidney Disease

Patients with existing hypertension, heart failure, heart disease, liver disease, or kidney disease should use Hydroxocobalamin with caution. Let your doctor know about all of your medical conditions before starting treatment.

How to Manage Common Side Effects

Most side effects don't require treatment, but here are some practical tips:

• For injection site pain: Apply a cold pack for 10 to 15 minutes after your injection. Gently moving the muscle can also help distribute the medication and reduce soreness.
• For red urine: Nothing to do — just know it's normal. If it helps, remind yourself before each dose so you're not startled. This can last up to 5 weeks.
• For nausea: Eat a light snack before your injection. Stay hydrated. If nausea persists, let your doctor know.
• For headache: Over-the-counter pain relievers like acetaminophen or ibuprofen are usually fine. Ask your doctor if you're unsure about drug interactions.
• For skin redness: Avoid direct sunlight and tanning beds while your skin is discolored. Use SPF 30+ sunscreen if you need to be outdoors.

Does Hydroxocobalamin Interfere with Lab Tests?

Yes — and this is important to know. The red color of Hydroxocobalamin in your blood can interfere with certain laboratory tests for up to 24 to 48 hours after a dose. Affected tests may include liver function panels, blood glucose, and other colorimetric assays. If you're having blood work done, let your healthcare provider know you're taking Hydroxocobalamin so they can account for possible interference.

Final Thoughts

Hydroxocobalamin is generally a safe and well-tolerated medication. The most common side effects — injection site soreness and red-colored urine — are harmless and temporary. Serious side effects are rare but important to recognize.

If you're just starting Hydroxocobalamin, don't let the possibility of side effects discourage you. B12 deficiency can cause serious problems if left untreated, including permanent nerve damage. The benefits of treatment almost always outweigh the risks.

Having trouble finding Hydroxocobalamin at your pharmacy? Use Medfinder to check which pharmacies near you have it in stock, or read our guide on how to find Hydroxocobalamin in stock.