Why is it important?
Because so many drugs (including nutrients and herbal therapies) are metabolised via the CYP450 system. This system can be inhibited or induced by drugs. And that then leads to drug-drug interactions and all sorts of adverse reactions. Drugs that cause CYP450 drug interactions are referred to as either inhibitors or inducers.
- Inducers can increase the rate of another drug’s metabolism by x2/x3 fold over the period of a week! So, when an inducing agent is prescribed with another drug, the dosage of that other drug may need to be increased. Otherwise, the HIGHER rate of metabolism means the medication doesn’t last long and its therapeutic effect is reduced leading to therapeutic failure of that medication.
- Inhibitors can decrease the rate of another drug’s metabolism by x2/x3 fold over the period of a week! So, when an inhibitor agent is prescribed with another drug, the dosage of that other drug may need to be reduced. Otherwise, the LOWER rate of metabolism means the medication last longer and its therapeutic effect is enhanced leading to harmful adverse effects.
And what makes it worse, is that there is genetic variability in about 7% of people. For example, many clinicians believe that post menopausal women recieving tamoxifen for early breast cancer should be tested for their type of CYP2D6 genotype as it may be valuable in selecting the type of adjuvant homonal therapy to offer. And of course CYP2D6 inhibitors should be avoided in tamoxifen-treated women.
And patients come to us with often co-morbidity. It would be nice if the world was just full of people with just the one thing. But it isn’t. It’s a lot more complex. It’s not uncommon for a patient withy hypertension and bad lipids to present to us at some point with depression, for instance. Did you know that several antidepressants are CYP450 inhibitors (i.e. reduce metabolism of other drugs). The effect on drugs like halperidol or metoprolol will then be accentuated.
In the case of grapefruit juice, there are numerous medications known to interact with grapefruit juice including statins, antiarrhythmic agents, immunosuppressive agents, and calcium channel blockers. Furthermore, the inhibition of the enzyme system seems to be dose dependent; thus, the more a patient drinks, the more the inhibition that occurs. Additionally, the effects can last for several days if grapefruit juice is consumed on a regular basis. Luckily, the effect of this is not seen with other citrus juices.