Concern over drug interactions with other drugs is based upon the danger that may be present when two drugs are combined. In some instances, taking two different drugs that are used for two different things make cause similar responses. If the response of one slows the heart, the second may dangerously slow the heart, breathing, etc. It could cause death.
Many years ago, there was a famous case of a young woman named Karen Ann Quinlan. I remember this vividly. She had taken two different kinds of drugs and alcohol as well. She stopped breathing several times. She was taken to the hospital—she had lapsed into a coma and doctors could not bring her out of her "vegetative state." Her parents, after long months of waiting, with no hope in sight, asked the doctors to remove her from the machines that kept her body alive. The hospital refused and the disagreement ended up in court where the "right to die" question was strenuously debated. Ultimately, the parents won their fight, but even after the machines were turned off, Karen remained alive (while her mental state never improved) for nine years until she died because of complications from pneumonia. This was a precedent-setting case.
The implications for drug users on any drug comes with risks. Watch and listen to many commercials or read the warning on pill bottles. Even the birth control pill introduces an elevated risk of blood clots. However, when drugs are mixed, the chances for drug interactions rise. This is why drug stores keep your medications on record and are supposed to let you know if there is a danger. Often warning labels will be placed on bottles to provide special instructions. Keeping your doctor advised of all medications you are taking, especially if you are under the care of another doctor (a specialist, for instance) is of paramount importance, as he or she can then monitor your risk of drug interaction.
Of course, if you are unsure, you can always contact the drug store and ask to speak to a pharmacist. I would hesitate to use the Internet only because of the ease with which people can provide information that may not be accurate.
What a drug may do by itself is one thing. A drug mixed with another drug because a totally new "entity." For some people, the mixture could be like taking poison.