Which of the following side effects requires immediate medical attention when taking Valacyclovir?

Shortness of breath is a significant side effect that necessitates immediate medical attention when taking Valacyclovir. This symptom can indicate a serious allergic reaction or other serious complications that may arise from the use of the medication. Respiratory issues can escalate rapidly and may pose a risk to the patient's well-being.

On the other hand, headaches, mild rashes, and insomnia, while they may be uncomfortable, typically do not signal an urgent medical situation. These side effects are more common and generally manageable without immediate intervention. Monitoring these symptoms is important, but they usually do not warrant the same level of urgency as shortness of breath, which can lead to respiratory distress and requires prompt evaluation.