Common side effects include headache, feeling sick, dizziness, vomiting, diarrhea, skin reaction after exposure to sunlight (photosensitivity), rash, or itching (pruritus).
Uncommon side effects include feeling confused, seeing or hearing things that aren’t there (hallucinations), feeling very drowsy, tremors, or feeling agitated, shortness of breath (dyspnea), stomach discomfort, rash, sometimes itchy, hive-like rash (urticaria), low back pain (kidney pain), blood in the urine (hematuria), reduction in the number of blood platelets which are cells that help blood to clot (thrombocytopenia), reduction in the number of white blood cells (leucopenia), increase in substances produced by the liver
Rare side effects include unsteadiness when walking and lack of coordination (ataxia), slow, slurred speech (dysarthria), fits (convulsions), altered brain function (encephalopathy), unconsciousness (coma), confused or disturbed thoughts (delirium), kidney problems where you pass little or no urine. Lastly, watch out for a severe allergic reaction (anaphylaxis). These are rare in people taking Valtrex. Anaphylaxis is marked by the rapid development of flushing, itchy skin rash, swelling of the lips, face, neck, and throat—causing difficulty in breathing (angioedema), fall in blood pressure leading to collapse. If any of these occur, get emergency treatment immediately