Taking Cephalexin when you do not have a bacterial infection may reduce the effectiveness of antibiotics when you really need them. Cephalexin may also be less effective if you do not take the full dose or cycle that your doctor has prescribed. I was prescribed cephalexin for a uti. Within 24 hours I had severe stomach pain and had to stop taking it. After four days of pain a blood test indicated pancreatitis and I was admitted to hospital. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take cephalexin exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor. Before you take cephalexin, inform your doctor about any previous adverse reactions you have had to medications, especially cephalosporins and penicillins. Contact your doctor at once if you experience signs of an allergic reaction, such as skin rash, itching, difficulty breathing, or swelling of the face and throat. Keflex can be taken with food or on an empty stomach. If the medication upsets your stomach, you should try taking Keflex with some food or milk. If you're unsure how to take your specific medicine or whether you can have anything to eat or drink before taking it, speak to your GP or pharmacist (chemist) or phone NHS 111 for advice. As a general rule, medicines that are supposed to be taken on an empty stomach should be taken about an hour before a meal, or 2 hours after a meal. Drugs.com provides accurate and independent information on more than 24,000 prescription drugs, over-the-counter medicines and natural products. Helpful, trusted answers from doctors: Dr. Ein on taking antibiotics on an empty stomach: Gi side effects such as nausea, vomiting, and abdominal pain are common with doxycycline. Suggest taking it with food to see if that helps. You don't have to. It would be pretty hard to take it on an empty stomach since you have to take it several times a day.