Generalized itching, especially on your legs, may be a sign of a more serious disorder known as exercise-induced anaphylaxis. The disorder is rare, but can be fatal. If you stop exercising after the onset of symptoms, in most cases you should be able to recover without requiring immediate medical attention. However, you should still see a doctor for diagnosis and treatment if you suspect that you might have this condition.  Symptoms to watch for include lightheadedness, sudden loss of muscle control, a tightness or constricted feeling in your throat, and difficulty swallowing or breathing. Your symptoms may be relatively mild, to the point you can comfortably ignore them and continue your exercise. If they increase in severity, however, you should stop running. With mild symptoms, it's possible they will improve if you slow down and take a break, and you may be able to resume your run without any problems. If symptoms require you to stop running, move to a protected area and set upright. Practice deep breathing and relax your muscles. In time, you should start to feel better.  Breathe in slowly through your nose, exhaling through your mouth. When your breathing feels regular, try to drink some water. Keep in mind symptoms may continue for hours after they began. If your symptoms seem to get worse even after you've stopped exercise, seek emergency medical attention immediately. Don't continue to run if you manage to stabilize and your symptoms seem to go away. You might try walking, but if you start running again so soon after an episode, the symptoms could quickly return with increased severity. Your doctor will need to know everything possible about your episodes, including everything you do in the hours leading up to your run. The more information your doctor has, the better able they will be to identify potential causes of your reaction.  Document where you run, the time of day, the weather (if you're running outside), and how long you had been running when you noticed the first symptom. Take your pulse, if possible, or at least make an attempt to estimate your heart rate or the intensity of your exercise. Take an inventory of household products and toiletries you commonly use, as well as everything you consumed before your run. Even if you've already eliminated possible allergies to these things, your doctor still may want this information. If you've recently changed soaps, detergents, or other products in an attempt to stop the itching, write this down as well, along with whether you noticed any relief after the change. Include details about what you were wearing on your run, and whether your skin felt unusually warm before the symptoms started. Understanding the symptoms you have is important for your doctor to figure out how to treat your reaction. Write down everything as soon as possible after it happens, even things you think aren't really symptoms or aren't significant.  Symptoms can vary greatly, which not only means that many people remain unaware of their condition, but also that doctors lack the information to diagnose them properly. Generalized itching, particularly when accompanied by welts or hives, can be more common. A tightness in your throat and difficulty breathing or swallowing is a common symptom of anaphylaxis, but it may not be a symptom you have. Other symptoms include nausea, low blood pressure, sudden loss of muscle strength or motor control, fainting, lightheadedness, and headaches. Exercise-induced anaphylaxis may be caused by a mild allergy you have to something else. This could include an allergy to shellfish, wheat, or any number of other food or drug products.  Your allergy may be so mild that you don't even realize you have it until you start exercising shortly after exposure to the antigen. Your increased temperature and heart rate as a result of exercise causes you to have an exaggerated reaction. However, you won't know if this is the cause unless you get tested for these common allergies. If allergy tests reveal the cause, you have a simple way to stop your legs from itching when you run – avoid exposure to whatever triggers an allergic reaction. Prescription antihistamines also may help you, but you'll need to talk to your doctor about which drugs are safe for chronic use. Exercise-induced anaphylaxis is a rare but serious condition, and episodes can be difficult to predict. If your doctor diagnoses you with this condition, you'll need to make arrangements so you can continue to run without endangering your life or well-being.  Your doctor will teach you about preventative measures you can take to avoid another episode, and may instruct you to wear a med-alert bracelet. You also may need to carry an epinephrine auto-injector with you when you run in case you need to thwart an episode. If you are diagnosed with exercise-induced anaphylaxis, you should avoid exercising alone, even if your symptoms are controlled or you go for a long period of time without experiencing any symptoms. Keep in mind this doesn't mean you can never run again. Part of the nature of exercise-induced anaphylaxis (if that is your ultimate diagnosis) is that it comes and goes. You may have no symptoms for months or even years, and then suddenly have another episode.
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One-sentence summary -- Stop exercising immediately if you notice lightheadedness or difficulty breathing. Relax and stabilize your breathing. Keep a diary of your episodes. List your symptoms. Get tested for allergies. Work with your doctor.


The benefit of multivitamins is that most are designed to give you the Recommended Daily Amount (RDA) of many different vitamins and minerals. The RDA is an amount that should be sufficient, but not too much, for most healthy adults.  Examine the label on the product. There should be a table that tells you what percent of the RDA the product has for each vitamin or mineral it contains. The best ones provide you with approximately 100% of the daily value for lots of different vitamins and minerals. If your doctor feels it would be beneficial, you can purchase multivitamins over-the-counter at drug stores and grocery stores. If the label on the bottle says that it provides much more than 100% of the recommended daily amount, then it is a megadose. For example, 500% of the RDA is a megadose. Over dosing on some vitamins can actually be harmful:  Both too little and too much vitamin B6 can cause nervous system problems. Fat soluble vitamins (A, D, E, K) are easier to overdose on because excess amounts are not excreted through the urine, as with water soluble vitamins. Too much vitamin A can raise the risks of hip fractures. Too much vitamin D can cause the blood to have too much calcium. This can cause vomiting and constipation. Overdosing on iron can cause vomiting and liver damage. Vitamins and minerals are frequently added to processed foods and beverages. If you are taking vitamins that provide a high amount of some vitamins, be aware that you may need to reduce your supplement intake if your diet already provides you with the right amount. Vitamins can degrade over time. This is particularly likely if they are stored in hot, humid places. If your vitamins have expired, it is safer and healthier to simply purchase new ones. If the type you are considering does not have an expiration date on it, don’t take it. The contents of vitamins and supplements are not strictly quality controlled the way food is. This means that it is difficult to be sure exactly what is in the pills that you are purchasing. Check with the Food and Drug Administration’s website to see if your supplements are under review. The website can also tell you if people have complained of negative reactions.
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One-sentence summary --
Consider a multivitamin. Don’t take large doses of any particular vitamin. Don’t take expired vitamins. Research the vitamins you are considering.