Vitamin B-12 and Phentermine

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Phentermine is a prescription weight-loss drug that does not interact with vitamin B-12 and does not have any pharmacological association with the nutrient. But as Mayo Clinic dietitian Katherine Zeratsky explains, weight-loss clinics sometimes include B-12 supplementation in their programs, claiming that the vitamin accelerates your metabolism. Zeratsky says that scientific evidence does not support this claim. If you are on phentermine, do not take B-12 without talking to your doctor.


Phentermine affects your nervous system in a way that suppresses your appetite. The drug does not cause weight loss, but it makes you want to eat less. This medication, available as tablets and capsules, is prescribed in conjunction with a regular exercise program and a low-calorie diet. But phentermine can be addictive, so your doctor may limit your prescription to a total of six weeks.

Composition of Phentermine

Vitamin B-12 does not appear in either phentermine tablets or capsules. The active ingredient in both forms is phentermine hydrochloride, accompanied by a number of inactive ingredients. The capsules also contain corn starch, gelatin, lactose monohydrate, magnesium stearate, titanium dioxide and black iron oxide, as well as the coloring agents Blue 1, Red 40 and Red 33. The tablets have a different inactive ingredients: corn starch, lactose, magnesium stearate, microcrystalline cellulose, pregelatinized starch, sucrose and Blue 1.

Vitamin B-12

Vitamin B-12 is available naturally in meats, eggs and dairy. You also find it as an over-the-counter and prescription supplement to correct a deficiency. B-12’s function is not to speed up your metabolism to make you burn calories fast. The nutrient facilitates the production of red blood cells and the operation of your nervous system. This vitamin is also essential for the making of DNA, the carrier of your genetic information.

Vitamin B-12 and Phentermine

If your doctor prescribes vitamin B-12 while you are on phentermine, she probably identified a deficiency of the nutrient in a lab test. Since the weight-loss drug suppresses your appetite, you might not be getting an adequate amount of the vitamin. Prevent a lack of B-12 by following your doctor’s dietary recommendations when she starts you on phentermine. If you follow a vegan diet, you may also need supplementation, since the nutrient is only naturally available in foods of animal origin. Some manufacturers enrich their products with B-12, but if you do not eat those particular foods, you do not get the nutrient. Low levels of the vitamin cause a form of anemia, as well as fatigue and neurological problems. A deficiency may also result in weight loss. However, your doctor would probably remind you that vitamin deprivation is not the ideal path to healthy weight.