Cold Sores

The usual dose for cold sore treatment is one 200 mg tablet taken five times a day for 5 days 

Allergy to aciclovir or valaciclovir or any of the other ingredients of this medicine

Common (up to 1 in 10 people)

• headache

• feeling dizzy

• feeling or being sick

• diarrhoea

• stomach pains .

• rash

• skin reaction after exposure to light (photosensitivity)

• itching

• feeling tired

• unexplained fever (high temperature) and feeling faint, especially when standing up.

Uncommon (up to 1 in 100 people)

• itchy, hive-like rash

• hair loss.

Rare (up to 1 in 1,000 people)

• effects on some blood and urine tests

• increases in the enzymes that work in the liver.

Very rare (up to 1 in 10,000 people)

• reduced numbers of red blood cells (anaemia)

• reduced numbers of white blood cells (leukopenia)

• reduced numbers of blood platelets (cells that help blood to clot) (thrombocytopenia)

• feeling weak

• feeling agitated or confused

• shaking or tremors

• hallucinations (seeing or hearing things that aren’t there)

• fits

• feeling unusually sleepy or drowsy

• unsteadiness when walking and lack of coordination

• difficulty speaking

• inability to think or judge clearly

• unconsciousness (coma)

• paralysis of part or all of your body

• disturbances of behaviour, speech and eye movements

• stiff neck and sensitivity to light

• inflammation of the liver (hepatitis)

• yellowing of your skin and whites of your eyes (jaundice)

• kidney problems where you pass little or no urine

• pain in your lower back, the kidney area of your back or just above your hip (renal pain)

• Valaciclovir

https://www.medicines.org.uk/emc/files/pil.4334.pdf