Discussions that mention cimetidine

Birth Control board


This is an effect that still needs to be studied, but first I'll give the cause and then the explanation.

The cause of the interaction is the ability of SJW to induce cytochrome P450, a class of enzymes that acts to metabolize (or break down) many different drug classes.

So what does that mean? Many drugs must be broken down by enzymes in the liver in order to be properly excreted (this is called drug metabolism). The enzymes that do this (again, located in the liver) are called cytochrome P450 enzymes. They are commonly named with the prefix CYP and then a series of letters and numbers (CYP2A1, CYP3A4, etc).

Doctors and pharmacists are trained to know exactly which enzymes metabolize which drugs. This is because there are some drugs that cause these enzymes to work EXTRA well or LESS well. Some of these drugs are phenobarbital, cimetidine, cigarette smoking, and many, many more. SJW is another of these drugs.

So lets look at an example. You are taking a drug like warfarin (aka Coumadin, a blood thinner). Warfarin is metabolized by CYP1A2. You also take cimetidine (aka Tagamet, an OTC antacid) for heartburn. One property of cimetidine is that it INHIBITS CYP1A2 - this means that the enzyme cannot do it's job of breaking down warfarin. The result is that you have too much warfarin (i.e. too high a dose, because the enzyme could not break it down), and hence your blood is now too thin. This is a common drug interaction.

Let's now look at your example (the opposite of the example we just looked at). You are taking birth control, which is metabolized by CYP3A4. You decide to take SJW at the same time. Researchers believe that SJW might be an INDUCER of the CYP3A4 (i.e. it causes the enzyme to work TOO WELL) - this will mean that the enzyme breaks down way more birth control than it should, and the end result will be that your body has LESS birth control than it needs to prevent pregnancy. I'm sure you can guess the end result of THAT...

Incidentally, this is exactly why you should never take antibiotics without using a second form of birth control (condoms) - the antibiotics induce these enzymes, and the BC dosage is lowered.

In fact, probably about 60% or so of drug-drug interactions are due to these enzymes.

So can you take SJW with birth control? You have to remember that SJW (most herbals, in fact) are not very well studied. We know that they are very safe on their own, but it is still not clear whether they induce these enzymes enough to cause appreciable interactions. Research takes time, and it will be a while before the SJW interactions are deduced. My advice with herbals (in terms of interactions, at least), is to always play it safe - use other forms of birth control in addition to pills.