Recently I had a client who arrived in his home country for his interview. He had an approved Form I-601A stateside waiver, no criminal record and all of his papers were in order.
Unfortunately he arrived in his country ill with flu like symptons. Being in a rural area, his mother obtained some medicine from a neighbor to help him feel better and so that he could make it to his medical exam. Big mistake! Whatever was in the local medication caused him to fail his medical exam! It showed up as an opiate in his results. He then had to see a psychologist and take a new medical exam which he passed. However once something like this happens it is not easily remedied. He was taken out of routine processing and his case was put aside for further review - a/k/a administrative processing.
The case has now been languishing over a month and a half and it could take as much as 8 months before this young man will finish processing and be allowed to return home.
Bottom line: Don't take medications unless they are under a doctor's order so that if something should show up, you will have a medical report to explain any unusual medical results and a doctor to back up your results. Many medications for stomach problems will often show up as any number of drugs. Better to put up with a little discomfort until you can return home than take a chance of being placed in administrative processing.