Definition
A drug interaction happens when two or more medicines, or a medicine and certain foods, affect each other so that a drug works too strongly, too weakly, or causes unexpected side effects. It can happen when absorption is blocked or when the liver and gut break a drug down faster or slower, which changes drug levels in your blood. Sometimes the drugs also overlap at the same target in the body.
Getting Stuck at the Gut's Front Door: A Roadblock
Picture two trucks meeting head-on in a narrow alley, each blocking the other's way. Inside your body, different substances sometimes run into each other in tight spots and get stuck.
Some antibiotics, like tetracycline, are a good example. Metal ions, such as calcium in milk or magnesium and aluminum in antacids, cling tightly to the drug molecules.
The clumped-together pair doesn't dissolve well and can't pass easily through the gut wall. So the drug never makes it into your blood and is flushed out, leaving lower drug levels in your blood. Iron and calcium supplements can latch onto some drugs the same way, so this kind of interaction can happen between vitamins and medicine too.
Grapefruit: The Juice That Stops the Cleanup Crew
Grapefruit is the fruit that comes up most often when people talk about drug interactions. That's because a few of its compounds tie up an enzyme (CYP3A4) that breaks down drugs in the wall of your gut.
Normally, when a drug comes in, enzymes in your liver and small intestine break it down at a steady pace so your body can get rid of it. But when grapefruit compounds suppress that enzyme, a bigger share of the same dose slips into your blood without being broken down. This has been seen with some blood pressure and cholesterol drugs that this enzyme normally breaks down.
As a result, even if you take just the prescribed dose, the drug level in your blood can climb several times higher, and its effects can become too strong. Other substances do the opposite and rev the enzyme up. St. John's wort, a herbal ingredient, is known to increase this enzyme, so some drugs taken with it may be broken down too fast and end up at lower levels in your blood.
To Be More Precise: They Can Also Clash Where the Body Responds
Drug interactions aren't only about how much of a drug stays in your body. Sometimes they collide at the spot where the drug actually does its job, the target on your cells.
For example, some allergy medicines that cause drowsiness, sleeping pills, and alcohol all work by calming brain activity. Even if the level of each in your blood stays the same, their effects pile up in the same direction, and the slowdown can become far too strong.
On the other hand, two drugs that push in opposite directions can cancel each other out. So drug interactions typically happen in three places: absorption in the gut, the enzymes in the liver and gut that break drugs down, and the targets on your cells. They can also happen at other steps, such as when the kidneys clear a drug out. The first two change how much drug is in your body, and the last one changes what the same amount does.
Common misconceptions
It's fine to take medicine with juice or milk instead of water, as long as it's easier to swallow.
Depending on the drink, how a drug is absorbed and broken down can change. Calcium in milk can bind to some drugs and block absorption, and grapefruit juice can suppress the enzyme that breaks down certain drugs, raising their levels in your blood.
Drug interactions only happen when you take several prescription drugs at the same time.
They can easily happen with dietary supplements, vitamins, herbal remedies, and even everyday fruits or teas, not just prescriptions from the doctor.
Where you meet it
Calcium in milk binds to tetracycline-type antibiotics and keeps them from being absorbed.
Grapefruit juice suppresses an enzyme in the gut wall and raises the blood levels of some blood pressure drugs that the enzyme breaks down.
Sleeping pills and alcohol both calm brain activity, so their effects stack up in the same direction.
In one sentence
A drug interaction happens when another medicine, food, or supplement taken at the same time blocks absorption, changes the enzymes that break a drug down, or overlaps at the same target, so the drug's amount or effect changes.