PSA for everyone on GLP-1 medications: GET YOUR B12 AND VITAMIN D LEVELS CHECKED.

I just got my bloodwork back after 8 months on Ozempic and my B12 is 148 pg/mL (normal is 200-900) and my vitamin D is 18 ng/mL (should be above 30). My doctor said I'm clinically deficient in both and started me on supplements immediately.

The symptoms I was attributing to "normal GLP-1 side effects" — fatigue, brain fog, tingling in my hands, muscle weakness — were actually from B12 deficiency. I feel like an idiot for not catching this sooner.

Apparently GLP-1 meds can reduce B12 absorption because they slow gastric emptying and reduce intrinsic factor production. And vitamin D deficiency is common in anyone losing weight rapidly because it's fat-soluble and gets released erratically during fat loss.

Please please please ask your doctor to check these levels. It's a simple blood test. Don't just accept fatigue as "part of the process."

This is an excellent PSA and I wish more prescribers were monitoring for this. Let me add some clinical context:

B12 (Cobalamin):

• GLP-1 agonists have been associated with reduced B12 absorption in several studies, particularly with long-term use (6+ months)
• Metformin, which many GLP-1 users also take, ALSO reduces B12 absorption — so if you're on both, your risk is compounded
• Symptoms: fatigue, numbness/tingling, cognitive issues, anemia
• Sublingual B12 (methylcobalamin 1000-2000mcg) is preferred over oral tablets because it bypasses the GI absorption issue

Vitamin D:

• Very common deficiency in the general population; rapid weight loss exacerbates it
• Important for bone health, immune function, and muscle function — all critical during weight loss
• Supplement: D3 (cholecalciferol) 2000-4000 IU daily, taken with a fat-containing meal for absorption
• Get retested in 3 months to ensure levels are normalizing

Other nutrients to monitor on GLP-1:

• Iron — especially in menstruating women eating less red meat
• Folate — often low alongside B12
• Zinc — important for immune function and taste perception
• Magnesium — already commonly deficient, worse with reduced food intake

Request a comprehensive metabolic panel plus B12, D, folate, and iron/ferritin at minimum every 6 months while on GLP-1 therapy.

Oh my god. I just read the B12 deficiency symptoms and I have literally all of them. I've been telling my doctor I'm exhausted and she keeps saying "that's normal on semaglutide" and "give it time." EIGHT MONTHS of feeling like death and nobody thought to check my B12??

I'm calling my doctor's office first thing Monday morning and demanding bloodwork. This is infuriating.

DO IT. And if they push back, specifically request B12, vitamin D, folate, and a CBC. You have the right to request bloodwork. My doctor admitted she "doesn't usually check those" for GLP-1 patients which is honestly a gap in care.

The tingling in my fingers was getting scary — I was worried about neuropathy. Two weeks after starting B12 supplements and it's already improving. TWO WEEKS. All those months of suffering were unnecessary.

My daily supplement stack on Mounjaro, developed with my functional medicine doctor:

• B12 methylcobalamin sublingual — 2000mcg
• Vitamin D3 + K2 — 4000 IU / 100mcg (K2 helps direct calcium properly)
• Magnesium glycinate — 400mg (before bed, helps with sleep AND constipation)
• Omega-3 fish oil — 2g EPA/DHA
• Iron bisglycinate — 25mg (I'm a menstruating female, levels were borderline)
• Zinc — 15mg
• Electrolyte mix — daily

Total cost: about £35/month. Bloodwork every 4 months to make sure everything's in range. My energy completely turned around after getting deficiencies sorted.