Cabergoline or Vitamin B6 for Prolactin: What's the Difference

On sports forums, vitamin B6 is sometimes called a ‘soft substitute’ for cabergoline in lowering prolactin. In reality there is a gulf between them: cabergoline is a prescription drug with a proven action on pituitary receptors, whereas B6 is a vitamin that only participates in dopamine synthesis. The editorial team explains what the difference consists of at the level of mechanisms and evidence.

Prolactin and dopamine inhibition

Prolactin is a hormone of the anterior pituitary whose main function is to ensure lactation. In men and non-pregnant women its level is normally low. Elevated prolactin (hyperprolactinemia) suppresses GnRH secretion, as a result of which LH, FSH and sex hormones decrease. In women this manifests as menstrual cycle disturbances and infertility, in men as decreased libido, erectile dysfunction, and sometimes gynecomastia.

The secretion of prolactin, unlike that of most pituitary hormones, is under constant inhibition. The main inhibitory signal is dopamine, which hypothalamic neurons release into the pituitary portal system. Dopamine binds to D2 receptors on lactotrophs — the cells that produce prolactin — and suppresses its synthesis and release.

There are many causes of hyperprolactinemia: pregnancy, prolactinoma (a benign pituitary tumor), hypothyroidism, kidney failure, medications (antipsychotics, some antidepressants, metoclopramide), stress, and also macroprolactinemia — a laboratory phenomenon in which a low-activity ‘big’ form of the hormone circulates in the blood.

That is precisely why the clinical guidelines of the Endocrine Society (Melmed et al., 2011) emphasize: before treatment the cause must be established. It determines whether medications are needed at all and, if so, which ones.

Cabergoline: a direct D2 receptor agonist

Cabergoline is a derivative of ergoline, a long-acting dopamine D2 receptor agonist. It mimics the action of dopamine directly on the lactotrophs, that is, it works at the very point where prolactin production is inhibited. For its effect it is not necessary for the body to synthesize more of its own dopamine.

A distinctive feature of cabergoline is its very long half-life, which according to the official label is approximately 63–69 hours. Therefore the drug, unlike the older bromocriptine, is taken not daily but once or twice a week. In a clinical study by Webster et al. (1994) cabergoline normalized prolactin more often and was better tolerated than bromocriptine in women with hyperprolactinemic amenorrhea.

In prolactinoma cabergoline not only lowers the hormone level but in most patients also reduces the size of the tumor. That is precisely why the Endocrine Society guidelines recommend dopamine agonists, primarily cabergoline, as first-line treatment for a symptomatic prolactinoma, even a large one.

The official label for treating hyperprolactinemia provides for a starting dose of 0.25 mg twice a week with gradual titration under prolactin monitoring. The specific regimen is determined by the endocrinologist.

Vitamin B6: a cofactor, not a prolactin drug

Vitamin B6 is a group of compounds (pyridoxine, pyridoxal, pyridoxamine) whose active form is pyridoxal-5-phosphate (PLP). PLP is a cofactor of more than a hundred enzymes, in particular aromatic L-amino acid decarboxylase, which converts L-DOPA into dopamine and 5-hydroxytryptophan into serotonin.

Hence the logic of the popular idea: if B6 is needed for dopamine synthesis, then extra B6 should enhance dopamine inhibition and lower prolactin. However, in a person with normal vitamin B6 status the enzyme is already supplied with its cofactor, and the rate-limiting step of dopamine synthesis is a different reaction — the conversion of tyrosine into L-DOPA by the enzyme tyrosine hydroxylase, for which B6 is not a cofactor.

In other words, an excess of B6 does not ‘rev up’ dopamine synthesis beyond need. It can matter only in a genuine vitamin deficiency, which in people with a normal diet occurs rarely, although it happens in alcoholism, some chronic illnesses and with the use of certain medications.

It is also important that B6 acts at the stage of synthesizing the mediator, not at the receptor. If the cause of hyperprolactinemia is a tumor that responds poorly to natural dopamine, or medications that block D2 receptors, an increase in dopamine synthesis will not be able to overcome this blockade. Cabergoline, however, acts at the same point as dopamine, but far more powerfully and for longer.

What the studies say

The effectiveness of cabergoline in hyperprolactinemia has been proven in randomized studies and enshrined in international guidelines. It normalizes prolactin in most patients with micro- and macroprolactinomas, restores the menstrual cycle and fertility in women and sexual function in men.

Regarding vitamin B6 the situation is quite different. The notion of its ‘antiprolactin’ effect comes mainly from small studies of the 1970s–1980s, some of which showed a short-term reduction of prolactin and some the absence of any effect. The editorial team is not aware of any modern high-quality randomized studies that would prove a clinically significant reduction of prolactin by vitamin B6 in people with hyperprolactinemia.

Vitamin B6 was also studied for suppressing lactation after childbirth, but in modern practice it is not regarded as a reliable means for this purpose. The Endocrine Society guidelines on hyperprolactinemia (Melmed et al., 2011) do not mention vitamin B6 as a treatment option.

So, in terms of evidence these agents are not even in the same category: cabergoline is a standard of treatment, B6 is a hypothesis with limited historical support.

Risks and safety

The most frequent side effects of cabergoline are nausea, dizziness, headache, and orthostatic hypotension, especially at the start of therapy. Dopamine agonists can cause impulse control disorders: a pathological urge to gamble, shop, and hypersexuality. It is important for the patient and their close ones to know about these effects.

A serious risk of ergoline dopamine agonists is fibrotic damage to the heart valves. It was identified primarily in patients with Parkinson's disease who received cabergoline in doses significantly higher than those used in hyperprolactinemia (Schade et al., 2007; Zanettini et al., 2007). Therefore in long-term treatment the doctor may prescribe echocardiography.

Vitamin B6 with excessive prolonged intake can cause sensory neuropathy: numbness, tingling, impaired coordination. The classic publication by Schaumburg et al. (1983) described such cases with the intake of gram doses of pyridoxine, but later reports also concerned much smaller doses. The European Food Safety Authority (EFSA) in 2023 revised and lowered the tolerable upper intake level of B6 for adults.

• Cabergoline: only as prescribed, with monitoring of prolactin and, when indicated, of the heart.
• Vitamin B6: do not exceed the established upper intake levels, take into account B6 in all supplements.
• Numbness in the extremities while on supplements is a reason to stop taking them and consult a neurologist.

Editorial conclusions

Cabergoline and vitamin B6 affect the dopamine regulation of prolactin at different points and with different strength. Cabergoline is a direct, powerful and long-acting D2 receptor agonist with proven effectiveness even in prolactinomas. B6 is a cofactor of dopamine synthesis, extra intake of which, without a deficiency, is unlikely to significantly change the prolactin level.

Comparing them as a ‘stronger’ and ‘weaker’ version of one agent is incorrect. Hope placed in a vitamin can delay the diagnosis of a prolactinoma or another serious cause of hyperprolactinemia.

At the same time, cabergoline too is not a ‘safe pill’: it has serious side effects and is prescribed only after the cause of elevated prolactin has been established.

We also recommend reading our materials on for whom doctors prescribe cabergoline and in which cases B6 supplements are appropriate, on the prolactin and macroprolactin test, and on the comparison of letrozole with exemestane.

References

1. Melmed S, Casanueva FF, Hoffman AR, et al. Diagnosis and treatment of hyperprolactinemia: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(2):273–288.
2. Webster J, Piscitelli G, Polli A, et al. A comparison of cabergoline and bromocriptine in the treatment of hyperprolactinemic amenorrhea. N Engl J Med. 1994;331(14):904–909.
3. Schade R, Andersohn F, Suissa S, Haverkamp W, Garbe E. Dopamine agonists and the risk of cardiac-valve regurgitation. N Engl J Med. 2007;356(1):29–38.
4. Zanettini R, Antonini A, Gatto G, et al. Valvular heart disease and the use of dopamine agonists for Parkinson's disease. N Engl J Med. 2007;356(1):39–46.
5. Schaumburg H, Kaplan J, Windebank A, et al. Sensory neuropathy from pyridoxine abuse: a new megavitamin syndrome. N Engl J Med. 1983;309(8):445–448.
6. Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academies Press; 1998.
7. EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA). Scientific opinion on the tolerable upper intake level for vitamin B6. EFSA J. 2023.