Always read the ingredients on the label before taking this or any other medication to ensure that there are no elements that may elicit an allergic reaction. Stop taking this medication and consult a doctor if you experience symptoms associated with allergies. Seek immediate medical attention if you experience any of the following side effects: abnormal changes of urine quantities, lower back pain, blood clots, easy bruising or bleeding, extreme fatigue or disorientation. The following is a list of less serious side effects associated with Acyclovir: nausea, diarrhea, headache, vomiting, loss of appetite, abdominal pains, feeling light-headed, and the sudden swelling of hands or feet. Consult a doctor if symptoms persist or worsen over time. Note that this is not an all-inclusive list of side effects. Consult a doctor for an extended understanding of the possible risks involved with taking Acyclovir.