What side effects can this medication cause? Although side effects from valacyclovir are not common, they can occur. Chickenpox Patients should be advised to initiate treatment at the earliest sign or symptom of chickenpox. Because genital herpes is sexually transmitted disease, patients should avoid contact with lesions or intercourse when lesions and or symptoms are present to avoid infecting partners. Symptoms of serious allergic reaction may include: valacyclovir pharmacy rash, itching, swelling, severe dizziness, trouble breathing. Do not start, stop or change the dosage of any medicine before checking with them first. We comply with the HONcode standard for trustworthy health information. Less serious side effects may include: nausea, vomiting, diarrhea, stomach headache, dizziness, tired joint menstrual mild skin or stuffy nose, sore throat.