Many patients do not realize that many drug rashes occur with a latency period. What is meant by latency period is that there is a time distance between the onset of the rash and the use of the medication. Suspected allergy to a particular drug, the first use of the drug is often 3-5 days incubation period, or even longer 2 weeks, there are some special rashes even to trace back to within 1 month of the drug may be. If the allergy is caused by a second dose, it may occur instantly within 24 hours. Each of us should pay special attention to whether we have a history of drug allergy to avoid the next time we mistakenly take that drug and have another drug rash possibility. In addition, we should be aware that some drugs such as carbamazepine and allopurinol may have very serious or even fatal drug rashes, so if you must take these drugs, you must pay special attention to your skin condition and stop using them as soon as you suspect an allergy.