Are medication interactions much more likely to occur between two medications than among three?
I think you surely know the answer to this. The changes are greater the more meds you factor in.
But treating it merely as a math problem:
If the chances of any two meds interacting is, say, 10%, then the changes that there will be at least one reaction among three meds is at least 27.1%**. It will probably be more, since there's a non-zero change that all three could interact.
** A 10% chance two meds will interact means there is a 90% chance that those two meds will not interact. So, three meds means the chances of all meds not interacting is 90x90x90 = 72.1%. Which means the chances of two of the three interacting is 27.1%.
I was thinking that molecules in a mixture of two compounds could commingle at any time, whereas in most interactions three would take much more of a coincidence to do so simultaneously.
Oh, you mean if you take three medications, is it more likely for any two to interact than for all three to interact at once?
So long as you keep the number you're taking constant, basically, the more of those required to interact at once, the less likely of that interaction occurring (this will vary depending on exactly what medicines we're talking about).
But if you vary the amount you're taking, the more you take the more likely you will get some kind of reaction.
Might I add that this is a very basic look at probability and entirely useless without more information. If you're on medication then do not screw around with it unless your doctor says it's safe.
Warning noted. Ask doctor first.
...all other things being equal...
When people talk about drug interactions, they aren't talking about direct interactions of one drug molecule with another drug molecule, they're talking about the effects of those drugs on human physiology. So, yes, the more drugs you take, the more chance of drug interactions.
You also half to look out for food/herbal/medication interactions. Things like grapefruit and some cholesterol lowering drugs, Ginkgo biloba and blood thinners, St. John's wort and antidepressants, should be watched very carefully. Caffeine, and even cigarettes can interact with medicines.
Things can become exponentially more complicated with more medications involved. Some medications may block P450 enzymes (which metabolize medicines), thus if you took another medication, you could face the risk of getting much more exposure than intended since your enzymes are unable to metabolize the drug. You are basically overdosing on a normal dose of a drug. P450 inhibition is a major source of drug-drug interaction complications.
If you're worried about drug interactions with over-the-counter medication, don't take Tagamet (Cimetidine) at all. It interacts with just about everything. Zantac (Ranitidine) is a much better product.
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