Drugs that are incompatible with Grapefruit are also incompatible with Kava.
Grapefruit juice:
While kava has yet to be shown to block the absorption of certain drugs, the part we're interested in is how it interacts with the "key drug metabolizing enzyme in your body". They're speaking of the P450 group of enzymes.Both grapefruit juice and the fruit itself can interact with more than 50 drugs—such as cholesterol-lowering statins, high blood pressure medications, and allergy drugs—raising the risk of side effects and other problems. Here’s how to know if you can safely enjoy grapefruit with your medication.
Grapefruit and medication could cause problems in two ways. First, grapefruit can block a key drug-metabolizing enzyme in your body, which in turn could lead to an increase in the blood levels of certain drugs. If that happens, it increases the risk of experiencing a side effect from that drug. On the flipside, it can also block absorption of certain drugs in your intestines. In that case, you could have less of the drug in your bloodstream than what you need, so the drug might not be effective for its intended purpose.
Surprisingly, it doesn’t take much of the fruit for a grapefruit and medication interaction to happen: As little as 1 cup of juice or two grapefruit wedges can be enough to cause problems, according to the Food and Drug Administration.
https://www.consumerreports.org/health/grapefruit-and-medications-can-be-a-dangerous-mix/
Kava has the same action. It suppresses activity at the same locations.Cytochrome P450 enzymes are essential for the metabolism of many medications. Although this class has more than 50 enzymes, six of them metabolize 90 percent of drugs, with the two most significant enzymes being CYP3A4 and CYP2D6. Genetic variability (polymorphism) in these enzymes may influence a patient's response to commonly prescribed drug classes, including beta blockers and antidepressants. Cytochrome P450 enzymes can be inhibited or induced by drugs, resulting in clinically significant drug-drug interactions that can cause unanticipated adverse reactions or therapeutic failures. Interactions with warfarin, antidepressants, antiepileptic drugs, and statins often involve the cytochrome P450 enzymes.
https://www.aafp.org/afp/2007/0801/p391.html
As you can see in the quote above, the 2D6 was inhibited 73% and the 3A4 inhibition at 78%. Very significant and similar (62%) to the profile of inhibition seen with grapefruit.In the present work the inhibition of P450 enzymes by kava extract and individual kavalactones in human liver microsomes (HLMs) was investigated. Whole kava extract (normalized to 100 microM total kavalactones) caused concentration-dependent decreases in P450 activities, with significant inhibition of the activities of CYP1A2 (56% inhibition), 2C9 (92%), 2C19 (86%), 2D6 (73%), 3A4 (78%), and 4A9/11 (65%) following preincubation for 15 min with HLMs and NADPH; CYP2A6, 2C8, and 2E1 activities were unaffected.
https://www.ncbi.nlm.nih.gov/pubmed/12386118
If you have any questions about your medication, see what your doctor says about taking it with grapefruit juice. If it's "go", then kava is unlikely to cause any issue with the metabolism of that drug.
TL/DR: Avoid kava if your medicine says "do not take with grapefruit or grapefruit juice". The drugs don't break down in your system like they should and could lead to problems when kava is combined.