Lysine may help reduce cold sore flare-ups, but don’t use it if you’re pregnant, as there isn’t enough research to support its safety. It’s a good idea to ask your doctor before trying any treatment.

Cold sores, or fever blisters, result from a viral infection. These painful, fluid-filled blisters commonly appear on or near the lips in clusters or patches. Cold sores are caused by the herpes simplex virus type 1 (HSV-1, also known as oral herpes). HSV-1 can be spread through close contact, such as kissing.

There is no cure for a cold sore infection, but this condition can be treated with common antiviral medication, such as valacyclovir (Valtrex) or acyclovir (Zovirax). A medication you might take in addition to antivirals is lysine, an essential amino acid found in protein. It isn’t produced in your body, so you must consume it through an oral supplement or diet.

Common foods rich in lysine include:

Lysine is essential for human health, aiding calcium absorption and collagen formation. Many people consume enough lysine in their diet. But some — athletes or vegans specifically — may need to consume more. If you don’t have enough lysine in your body, you may experience irregular symptoms, including:

In order to replicate in the body, herpes simplex needs a steady source of arginine. This amino acid converts into nitric oxide within your body, a chemical necessary to expand blood vessels and improve blood flow.

Lysine is thought to interfere with arginine being absorbed into your intestine, making the amino acid unavailable for the virus. So even if it may not cure a cold sore infection, lysine can help slow or prevent the HSV-1 replication process.

In addition, older research suggests that lysine may reduce the number of occurrences of this infection and decrease healing time. For example, a small study from 2005 showed that lysine sped up the healing process for 87 percent of participants, shortening the average time from 21 days to 6 days for these people.

Lysine is available as an oral supplement and a cream.

To treat a cold sore infection, apply lysine cream to the affected area every two hours for 11 days.

To manage cold sore infections, the recommended dosage for oral supplements is 1 gram three times daily. To help prevent them, the dosage is 1 gram daily.

It’s also recommended that you take a zinc and vitamin C supplement to provide your immune system with an additional boost.

If you see no improvement in your condition, discuss dosages and alternative treatment with your doctor. If you begin to experience worsening symptoms, stop treatment immediately.

When taken at its recommended dosage, lysine is generally safe. However, it can cause a number of side effects, including:

There isn’t enough information about the safety or side effects of taking lysine during pregnancy. If you’re breastfeeding or pregnant, this supplement isn’t recommended. When incorporating lysine into your diet, eat foods in moderation to avoid weight gain or developing a health condition.

Before taking lysine or any other supplement, discuss the risks with your doctor. Lysine can interact with other medications you may be taking. It can also increase your calcium absorption, which can in turn weaken your bones and affect organ function.

A cold sore infection can’t be cured, but lysine treatment can reduce its recurrence. Though shown to be effective, lysine isn’t recommended for pregnant women since there isn’t enough information on how safe it is for these women.

Before pursuing treatment, discuss lysine and other alternatives with your doctor to determine the best treatment for your cold sores.