It's important to monitor your dog closely for any signs of an adverse reaction, whether it's from medication, food, insect bites, or other environmental factors. Here are some key signs that indicate you should take your dog to the vet:

1. **Swelling**: If you notice any swelling, especially around the face, ears, or throat, it could indicate an allergic reaction, which can be serious.

2. **Difficulty Breathing**: If your dog is having trouble breathing, is wheezing, or is making unusual noises while breathing, this is an emergency situation and requires immediate veterinary attention.

3. **Vomiting or Diarrhea**: Occasional vomiting or diarrhea might not be a cause for alarm, but if it’s severe or persistent, or if you notice blood, you should contact your vet.

4. **Lethargy**: If your dog suddenly becomes very tired or unresponsive, it could be a sign of a serious reaction.

5. **Hives or Rash**: Skin reactions, such as hives or rashes, can indicate an allergic response and should be evaluated by a vet.

6. **Seizures**: Any seizure activity is a medical emergency, and you should seek veterinary care immediately.

7. **Changes in Behavior**: If your dog is unusually agitated, anxious, or shows signs of pain (such as whining, pacing, or reluctance to move), it’s wise to consult your vet.

If you are ever in doubt about your dog's health or if you notice any concerning symptoms after a reaction, it's always best to err on the side of caution and contact your veterinarian for advice. They can help determine if your dog needs to be seen right away or if monitoring at home is sufficient.

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Topics covered in this question:

Dog Health Dog Care Pet Safety Vet Visit Allergic Reaction