That's right Crewman. It is well known in pharmacology that a 50%/50% mixture of two medicines can be far more powerful (or dangerous) than single 100% dose of one medicine.
Afaik, when medicines are licensed for use, they're not normally tested in combination with any other medicine. They're tested individually.
But if a pill contains more than one drug, then that hasn't necessarily been tested for the safety of it's combined effect. There are licences granted for each separate component if proven safe when taken on its own, but the license for the combined drug is granted automatic, even if the combination happens to be harmful !
Furthermore, when you get prescription medicines, they often list stuff that you're not supposed to take with it because of such reactions (contraindications). But in my experience, doctors don't pay any attention to it, even though they're supposed to.