( CNN ) -- A Singapore man undergoing treatment for cancer was detained for four hours by U.S. immigration officials after the drug he was taking caused his fingerprints to disappear . Foreign visitors undergo mandatory fingerprint screening when they enter the United States . Now , the unidentified man 's oncologist is asking patients taking the commonly used drug , capecitabine , to carry a doctor 's note when they travel to the United States . The oncologist , Eng-Huat Tan of the National Cancer Center in Singapore , described the encounter in a letter published in the current issue of the cancer journal Annals of Oncology . The 62-year-old cancer patient , identified as Mr. S , eventually was allowed to enter the United States and visit relatives after officials determined he did not pose a security threat . According to the oncologist , the patient had neck and head cancer that had spread . He responded well to chemotherapy . To prevent a recurrence , doctors placed him on capecitabine , marketed in the United States as Xeloda . One of the side effects of the drug is hand-foot syndrome . It causes the skin on the hands and feet to peel . With time , the drug can erase fingerprints . `` It is uncertain when the onset of fingerprint loss will take place in susceptible patients who are taking capecitabine , '' the doctor wrote . His patient started on the drug in July 2005 . `` However , it is possible that there may be a growing number of such patients as Mr. S. ... These patients should prepare adequately before traveling to avert the inconvenience that Mr. S was put through . '' Foreign visitors undergo mandatory fingerprint screening when they enter the United States .
U.S. immigration officials detain cancer patient who lacked fingerprints . Drug causes skin to peel , sometimes obliterates prints , doctor writes . Physician suggests affected travelers carry notes from doctors .