Our Keflex (cephalexin) Side Effects Drug Center provides a comprehensive view of available drug information on the potential side effects when taking this medication. This is not a complete list of side effects and others may occur. Typically, they should not make you drowsy, they never did me---but everyone is different. You can google it and check the side effects. Also, the Z-pack did not ever really cure my symptoms, but it did give me some nasty diarrhea--sorry.Also, you may be tired from whatever is ailing you--not the antibiotics. Can keflex make you sleepy - On around the class 1x6 boards to the after another gets it. Or an ebook you toll on our backs Paper Products at WallpapersWall them. If you're sick and you're feeling really, really tired, it may not be just the illness itself that has you dragging. It could be the antibiotics you're taking to cure it that is the problem. You must check to make sure that it is safe for you to take this medicine (cephalexin capsules and tablets) with all of your drugs and health problems. Do not start, stop, or change the dose of any drug without checking with your doctor. You should not use Keflex if you are allergic to cephalexin or to similar antibiotics, such as Ceftin, Cefzil, Omnicef, and others. Tell your doctor if you are allergic to any drugs, especially penicillins or other antibiotics. Some people with penicillin allergies (including amoxicillin, ampicillin, and others) can have a fatal reaction with Keflex. However, I know that Motrin also offers a cough syrup which can make you drowsy. Which is why most people administer it before bed. But other than that, no. Which is why most people administer it. Will you have Drowsiness with Cephalexin - from FDA reports Summary Drowsiness is found among people who take Cephalexin, especially for people who are female, 60+ old , have been taking the drug for 1 month, also take medication Lorazepam, and have Pain. Can Cephalexin cause Sleepy? Complete analysis from patient reviews and trusted online health resources, including first-hand experiences. Complete analysis from patient reviews and trusted online health resources, including first-hand experiences. Like any drug, cephalexin can cause allergic reactions. These may include hives, a mild rash, or angioedema -- swelling of the skin or the mucous membranes lining the inside of the mouth or throat.