For safety assurances, proper testing should be done for every drug combination you are advised to take. If you take Prozac and Tylenol, for example, you should be presented with all the possible benefits and consequences before allowing these two foreign substances to mix with the chemicals your body already creates. The same thing goes for combining Paxil with Viagra or Interferon with Lipitor.
The list of possible problems here is monstrously long, because there are a huge number of possible combinations. Nonetheless, there have been relatively few studies that test drugs in combination. So if you take two drugs, the odds of their combination having been adequately tested for safety are skimpy at best. But if you take three or more drugs the danger possibilities multiply even faster.
If you take 3 drugs then adequate safety testing of the various combinations require 7 separate tests. If you take 4 drugs the combinations require 25 separate tests. If you take 5 drugs it amounts to 121 tests. If you take 10 drugs the number of required safety tests total 362,881.
The conclusion here should be obvious. There is questionable safety testing if you take two drugs and nominal, if any, safety testing if you take three. Beyond that you are clearly into the land of, “I have no idea what these combinations of drugs will do.”