Most of the times, when you are allergic to a drug, an alternative drug is available. If not, the doctor may attempt to desensitise you or, in other words, make you tolerant to the drug again. In this desensitization procedure, you start with a minute amount of the drug and take a little more every 30 to 60 minutes until you can tolerate the amount you need to take. This may take one or two days or even more. This desensitisation procedure is, just like the drug provocation test, not without danger since you may develop your allergy symptoms. Therefore, this is normally done in hospital under strict supervision.
Furthermore, the state of tolerance stays as long as you take the drug daily. If you stop taking the drug or forget to take it a few days, you do lose your tolerance to the drug!