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  • L-Lysine

    I am currently nursing my 5 month old son and have developed a cold sore. Is it safe to take L-Lysine while nursing? Directions on bottle say to take it twice a day. Thank you!

  • #2

    This is all I know about Lysine, except that it probably doesn't work. I do not think it is overtly hazardous at low doses.

    Tom Hale PH.D.


    Lysine is a naturally occurring amino acid; the average American ingests around 6-10 grams daily. Aside from its use as a supplement in patients with poor nutrition, it is most often used for the treatment of recurrent herpes simplex infections, for which it is probably ineffective. The clinical efficacy of lysine in herpes infections is highly controversial with some advocates[1,2] and many detractors.[3] Upon absorption, most is sequestered in the liver, but blood levels do rise transiently. However, the risk of toxicity is considered quite low in both adults and infants. Rather high doses have been studied in infants as young as 4 months, with doses from 60 to 1080 mg L-lysine per 8 ounces of milk.[4] At the higher level, 5.18 grams of L-lysine was consumed. Plasma lysine levels varied only with normal limits, while urinary lysine levels were roughly proportional to the amount of supplementation. Thomas has reported an elegant study of the transfer of radiolabeled L-lysine into numerous compartments, including milk.[5,6] In a group of 5 lactating women who received L-lysine (15N-lysine and 13C-lysine (5 mg/kg/each)), milk levels of labeled lysine reached a peak at approximately 150 minutes. Labeled lysine levels in milk were slightly higher with M/P ratios ranging from 1.29 to 1.43. However, the total amount of radiolabeled lysine present in milk was still low. Only 0.54% of the administered dose of lysine was secreted into milk proteins. Further, the lysine present in milk was present as protein, not free amino acid.

    Therefore supplementation of breastfeeding mothers with L-lysine will probably not result in significantly elevated levels of free lysine in milk.