Clinically, to know if a person is allergic to penicillin, one needs to first take a medical history. If there is a prior history of allergy to many types of foods or medications, there may be an allergy to penicillin. If there is no obvious history of cephalosporin or penicillin allergy, further determination at this point can be made by choosing to do a skin test, which involves taking a small amount of penicillin solution and injecting it into the body to observe for immune reactions such as redness, swelling, and hard nodules. If the skin test is positive, this will then indicate an allergy to penicillin. If the skin test is negative, treatment with penicillin can be continued, but there is no 100% guarantee that you will not be allergic to penicillin when it is applied, so you should still observe accordingly. If there is no obvious allergy such as chest tightness, rash, or itchy skin at the time of injection and for a period of time after the injection, then it will prove that there is no allergy to penicillin.