Think ahead. If, for example, you are a diabetic, bring snacks that will keep you from going into insulin shock. If you have asthma, bring your medications and inhalers so you can avoid an attack.
Analyze your symptoms. If you feel a headache coming on, or are stuffed up because of allergies, you may be able to alleviate these symptoms with over-the-counter medication.
Wait until the next break if you need to take any medications. If it can't wait, talk to the bailiff or judge.
Speak with the bailiff or the judge immediately if you have symptoms indicating serious illness.
Follow the instructions of the court regarding whether you can return to jury duty when you feel better.
Remember that every jury panel has a few alternate jurors who are prepared to take the place of a juror who is ill or has a personal emergency.