Allergic drug rash:

Most allergic drug rashes start within two weeks of taking a new medication, especially if the person has taken the drug before. It is very unlikely for medicine that has been prescribed for months or years to cause an allergic reaction. Because there is usually no specific test to prove whether a rash is allergic, doctors may recommend stopping a suspected drug to see what happens. If the rash doesn't disappear within five days of not taking the medication, allergy is unlikely.

Although foods, soaps, and detergents are often blamed for widespread rashes, they are rarely the culprit.