My Boyfriend Takes A Lot Of Medications For Mental Illness

My boyfriend takes a lot of medications for mental illness and is also suffering from serious back pain which has him on different pain meds as well. He just took a swab test at the doctors and they are saying that he has tested positive for cocaine. Over this last weekend, I applied lidocaine gel to his back that was prescribed by his pain management doctor and I can attest that he does not take any form of narcotics due to the large amounts of medications he takes. Is there something that could cause a false positive?