Vitamin D Adequacy Conditions the Prolactin-Suppressive Effect of Metformin in Men Receiving Prolactin-Elevating Medications.

Plain-language summary

This study looked at whether vitamin D changes how well metformin (a common diabetes drug that lowers blood sugar) can bring down prolactin (a pituitary hormone that suppresses testosterone when it runs high) in men whose prolactin had been pushed up by antipsychotic medication. Seventy-five men with high prolactin and prediabetes or type 2 diabetes were grouped by vitamin D status, sufficient on their own, deficient, or sufficient thanks to supplements, and all took metformin for six months. Metformin lowered prolactin only in the men with adequate vitamin D, and in those men testosterone rose a little and sexual function improved.

Key findings

• Metformin reduced prolactin levels only in men with sufficient vitamin D, whether naturally or from supplements.
• Men with sufficient vitamin D also saw modest increases in LH and testosterone, and improvements in sexual function.
• Vitamin D levels did not change during the study, and the effect of metformin did not differ between naturally sufficient and supplemented men.