L-LysineAmino Acid

• Cold sore recurrencePossibly helps

  The 2015 Cochrane systematic review of 32 randomized trials on measures against recurrent cold sores found no evidence of a preventive effect for lysine [7].
  
  A randomized placebo-controlled crossover trial did find 1,248 mg/day reduced recurrence, while 624 mg/day did not [6]. Neither dose shortened how long lesions lasted [6].
  
  An earlier prophylaxis trial at 1,000 mg/day over 6 months also reported fewer recurrences than placebo [9].
  
  The Natural Medicines database rates oral lysine possibly effective here, at 500–1,248 mg/day or 1,000 mg three times daily [5]. ^[7]

• Cold sores — the sources genuinely disagreePossibly helps

  This one does not resolve, and we are not resolving it for you.
  
  A 2017 narrative review concluded lysine looks ineffective below 1 g/day without an accompanying low-arginine diet, while above 3 g/day patients report better subjective symptoms [8].
  
  The same review calls for longer controlled trials above 1.2 g/day, and says no convincing evidence supports using lysine on an already-active lesion [8].
  
  Cochrane sits at the higher evidence tier and found nothing; the narrative review re-analysed an overlapping, mostly small and low-quality trial set by dose [7][8]. ^[8]

• Canker sores (aphthous ulcers)Possibly helps

  The Natural Medicines database rates the evidence here as insufficient to rate [5].
  
  Early research suggested 500 mg/day may reduce recurrence and 4,000 mg/day may shorten how long an ulcer lasts [5]. ^[5]

• Anxiety and stressPossibly helps

  A 3-month randomized double-blind trial fortified wheat flour with lysine in lysine-deficient rural Syrian communities [11].
  
  Chronic trait anxiety fell in men, and the acute cortisol and sympathetic response to a blood draw was blunted in women and children [11].
  
  A separate placebo-controlled trial gave healthy men oral L-lysine plus L-arginine for one week, measuring lower state and trait anxiety and lower basal salivary cortisol [12]. ^[11]

• Why lysine might affect anxiety — rat work onlyPossibly helps

  The proposed mechanism comes from rats: lysine acts as a partial serotonin 5-HT4 receptor antagonist, blunting 5-HT4-mediated stress and anxiety responses [10].
  
  Dietary lysine deficiency in rats separately worsened stress-induced anxiety, through increased serotonin transmission in the amygdala [10].
  
  That is animal evidence. The human trials above are consistent with it but are small field and short supplementation studies, not large confirmatory RCTs [11][12]. ^[10]

• Hair thinningPossibly helps

  One study added L-lysine to iron supplementation in women with chronic diffuse hair thinning who had not responded to iron alone, and serum ferritin rose significantly [29].
  
  The proposed route is better iron and zinc uptake, not a direct action on hair [29].
  
  The reviewing authors call the data limited and lysine's role in need of further investigation [29]. ^[29]

• Decreased appetite
• Weight loss in adults
• Growth delay in children
• Anemia
• Fatigue
• Mood changes such as irritability and agitation^[13]

• L-lysine hydrochlorideNearly every commercial supplement — chosen for stability, water solubility and manufacturing consistency

  Elemental content

  About 80% free lysine by weight — 182.65 g/mol against 146.19 for the free amino acid [16]

  Absorption

  A label reading 1,000 mg of L-lysine HCl delivers roughly 800 mg of the actual amino acid [16].
  
  Some products state free-lysine content and others the total salt weight, which makes matching a trial dose harder than it looks [16].

  Best for

  Nearly every commercial supplement — chosen for stability, water solubility and manufacturing consistency [16]

• Free-form L-lysineNo documented advantage over the HCl salt once you adjust for lysine content

  Elemental content

  The unsalted amino acid, 146.19 g/mol [16]

  Absorption

  More hygroscopic and more prone to degrading in humid storage than the hydrochloride [16].
  
  No controlled human trial has compared the two directly; sources describe no established difference once actual lysine content is matched [16].

  Best for

  No documented advantage over the HCl salt once you adjust for lysine content [16]

• Topical lysine gels and creamsNot a stand-in for the oral form — unlike topical docosanol, topical lysine is not FDA-approved for this use

  Elemental content

  Lysine is typically listed as an inactive ingredient [16]

  Absorption

  The actual active ingredients are usually antiseptics or analgesics such as benzalkonium chloride or menthol [16].
  
  No good clinical evidence shows topical lysine is meaningfully absorbed through skin or acts on the virus at the application site [16].

  Best for

  Not a stand-in for the oral form — unlike topical docosanol, topical lysine is not FDA-approved for this use [16]

• Headache and gut symptoms at high doses

  Headache and gastrointestinal symptoms — nausea, vomiting, abdominal pain, diarrhea — appeared around 6 hours after a 7,500 mg dose in one study [18].
  
  A commercial medical reference separately lists abdominal pain and diarrhea among lysine's adverse effects [25]. ^[18]

• Kidney and liver failure

  A 2022 review in the British Journal of Clinical Pharmacology states lysine supplementation is contraindicated in hepatic or renal failure [18].
  
  It advises against long-term dosing at 3,000 mg/day or above because of a chance of kidney damage, and puts the safest range at roughly 500–3,000 mg/day [18]. ^[18]

• The kidney evidence pulls in two directions

  Norway's VKM toxicology assessment found no renal changes in a 90-day rat study even at 3,357 mg/kg/day [4].
  
  Medscape lists chronic renal failure and interstitial nephritis among reported human adverse effects, and flags kidney disease as a caution [25].
  
  Animal toxicology, human adverse-event reporting and clinical caution are three different kinds of evidence, and all three stay visible here [4][18][25]. ^[25]

• Inherited disorders of lysine metabolism

  Lysine supplementation is contraindicated in hyperlysinemia and hyperlysinuria, both rare inherited metabolic disorders [18].
  
  The same review recommends closer monitoring in people with sexual or erectile dysfunction, gallstones, asthma, or immune dysfunction [18]. ^[18]

• Pregnancy and breastfeeding — sources disagree

  Medscape categorises both pregnancy and lactation use of lysine supplements as avoid [25].
  
  The NIH's LactMed database calls lysine supplements probably a low risk while breastfeeding, adding that another approach may be preferable with a newborn or preterm infant [23].
  
  Dietary lysine requirements genuinely rise in later pregnancy, so the amino acid from food is not the concern — concentrated supplements without dedicated safety trials are [24]. ^[25]

• Calcium

  In a controlled human study, 400 mg of L-lysine given with an oral calcium load blunted the urinary calcium loss otherwise seen in healthy women [17].
  
  Short-term 800 mg/day raised intestinal calcium absorption in osteoporotic patients, where equal doses of valine or tryptophan did not [17].
  
  The BJCP review advises caution combining lysine with calcium supplements in people with heart disease, for exactly that reason [18]. ^[17]

• Aminoglycoside antibiotics

  The BJCP review notes lysine may increase the toxicity of aminoglycoside antibiotics [18].
  
  It separately flags an unresolved question about whether lysine could raise viral load in some HIV lineages, given high dietary lysine intake in some HIV-positive populations [18]. ^[18]

Lysine sits mainly in animal protein.

Using USDA FoodData Central figures: beef skirt steak about 3.31 g/100 g, lean chicken breast 3.08 g, lean pork chops 2.76 g, tuna 2.75 g, king crab 1.68 g [20].

Dairy and soy come next — low-fat ricotta 1.35 g/100 g, firm tofu 0.88 g, skim milk 0.28 g [20]. Canned navy beans run about 0.53 g/100 g [20].

Because lysine is the first-limiting amino acid in cereal grains, a plant diet built on wheat, rice or corn is where shortfalls appear [13].

Legumes, soy and quinoa close that gap: cooked lentils about 0.63 g/100 g, tofu 1.1–1.2 g, tempeh 0.9 g, soy flour around 3.1 g [21].

A raw-versus-cooked flag on quinoa: one table citing raw quinoa gives about 0.78 g/100 g, another measuring cooked quinoa gives about 0.24 g [21]. Cooking absorbs water and dilutes per-gram density, so this is a measurement mismatch rather than a database conflict.

Heat costs lysine through the Maillard reaction, where its reactive side-chain amine binds reducing sugars during baking, toasting or sterilisation [26]. Baking trials found roughly 15% lysine loss in bread, range 9.5–23.8%, plus another 5–10% from toasting [26].

Heat-processed milk products lost between 2.5% of available lysine in cooked cream and 36.2% in condensed milk, depending on heat intensity [27].

Groups flagged at higher risk of shortfall: people on cereal-staple or chronically low-protein diets, vegetarians, patients with chronic renal failure, and children with kwashiorkor [13]. Athletes with high training volumes and burn patients are named elsewhere, for their raised protein turnover [14].

No controlled trial has established a best time of day; the guidance in circulation comes from supplement and wellness sources, not clinical work [18][22].

Their consistent advice is to take it on an empty stomach or between meals. Lysine is absorbed by the y+ cationic amino acid transporter family, which it shares with arginine and ornithine, so a mixed-protein meal means competition [18].

The 2022 BJCP review says plainly there is still no consensus on fasting versus with food, and that lysine's pharmacokinetics may genuinely differ between the two [18].

Consumer sources also suggest separating lysine from calcium supplements at the same sitting, since lysine raises calcium absorption [22].

Plasma lysine peaks roughly 1–2 hours after an oral dose, dose-dependently [16]. One small older study found that keeping blood lysine above 165 nmol/mL, which took 1,000 mg/day or more, went with fewer outbreaks [16].