Some fruits, vegetables, and plant parts can cause real problems when juiced, from kidney damage to dangerous drug interactions to simply clogging your machine. While most produce is perfectly safe, a few common items deserve caution or should be avoided entirely. Here’s what to leave out of your juicer and why.
Rhubarb Leaves and Other Toxic Plant Parts
Rhubarb stalks are fine to juice, but the leaves are genuinely toxic. They contain about 0.5 grams of oxalic acid per 100 grams of leaf. While the estimated lethal dose of oxalic acid is 15 to 30 grams (meaning you’d need to consume kilograms of leaves), much smaller amounts can cause nausea, vomiting, abdominal pain, and burning in the mouth and throat. Juicing concentrates plant material quickly, so it’s easy to process far more leaves than you’d ever eat whole.
Apple seeds contain amygdalin, a compound that releases cyanide when broken down. A juicer can crush seeds that you’d normally spit out. The good news: freshly juiced apples contain very low cyanide concentrations, and one study estimated an adult would need to consume 16 regular-sized smoothies in under two hours to reach a lethal dose. Still, if you’re juicing apples daily, coring them first removes the risk entirely. The same logic applies to cherry pits, peach pits, and apricot kernels, all of which contain amygdalin in higher concentrations.
Spinach, Beet Greens, and Swiss Chard in Large Amounts
These three greens are the high-oxalate trifecta, and juicing makes it dangerously easy to overconsume them. Oxalate is the compound behind the most common type of kidney stone. When oxalate concentration in your urine gets high enough, it crystallizes out of solution and forms stones. There are documented cases of people developing kidney problems from juicing regimens that delivered more than 1,200 mg of oxalate per day, a threshold that’s shockingly easy to hit with spinach. Just two cups of raw spinach gets you there.
To put this in perspective, kale has hundreds of times less oxalate than spinach. You’d need to juice over 600 cups of kale to match the oxalate in two cups of spinach. Collard greens, mustard greens, and bok choy are similarly low-risk. If you love green juice, simply swap spinach for kale or another low-oxalate green. If you still want to use spinach occasionally, know that boiling it first cuts oxalate levels by more than half, though that obviously complicates a juicing routine.
Grapefruit If You Take Certain Medications
Grapefruit juice interferes with a key enzyme in your small intestine that helps break down dozens of common medications. When grapefruit blocks this enzyme, more of the drug enters your bloodstream and stays there longer, essentially giving you an unintended overdose. For a few other drugs, grapefruit has the opposite effect, blocking absorption so the medication doesn’t work well enough.
The FDA lists several drug categories that interact with grapefruit juice:
- Cholesterol-lowering statins like simvastatin and atorvastatin
- Blood pressure medications like nifedipine
- Anti-anxiety medications like buspirone
- Heart rhythm drugs like amiodarone
- Organ transplant rejection drugs like cyclosporine
- Some corticosteroids used for Crohn’s disease and ulcerative colitis
- Certain antihistamines like fexofenadine (where grapefruit reduces the drug’s effectiveness)
If you take any prescription medication, check the label or ask your pharmacist before making grapefruit a regular part of your juice rotation. This applies to whole grapefruit too, but juicing concentrates the effect since you’re consuming more fruit per serving.
Too Much Celery and Sun Exposure
Celery contains natural compounds called furocoumarins (including psoralens) that become toxic to skin cells when activated by UV light. For most people eating normal amounts of celery, this isn’t a concern. But the celery juice trend, where people drink multiple glasses per week, changes the math. One documented case involved a woman who drank at least four glasses of celery juice weekly for six months before a sun-heavy trip. High furocoumarin intake followed by UV exposure has been linked to severe phototoxic reactions, including burns, blistering, and swelling.
Limes and other citrus can cause similar reactions on the skin (the classic “margarita burn”), but celery juice is the bigger concern here because people drink it in such large, concentrated quantities. If you’re a heavy celery juicer heading into summer, this is worth knowing about.
Raw Cruciferous Vegetables in Excess
Kale, broccoli, cabbage, cauliflower, Brussels sprouts, and turnips all contain compounds called thioglucosides that break down into thiocyanates. These chemicals interfere with your thyroid’s ability to absorb iodine, which it needs to produce hormones. In animal studies, high doses of these compounds have caused thyroid problems.
The reassuring news: human studies haven’t shown a clear link between cruciferous vegetable consumption and thyroid disease in people with adequate iodine intake. The concern is really about volume. Eating a serving of broccoli with dinner is completely different from juicing several cups of raw kale every morning. If you already have a thyroid condition or low iodine levels, rotating your greens rather than relying on large daily doses of cruciferous vegetables is a sensible approach.
High-Sugar Fruits Without Fiber
Juicing strips out most of the fiber from fruit, and that fiber is what slows sugar absorption into your bloodstream. Apple juice produces a significantly larger insulin response than whole apples or even blended apples. When you juice fruits like grapes, pineapples, mangoes, or watermelon, you’re essentially drinking concentrated sugar water with vitamins.
Blending (which retains fiber) generally produces a lower blood sugar spike than juicing. If you’re making fruit-heavy juices, the glycemic impact adds up fast, especially if you’re combining multiple sweet fruits in one glass. A better approach is to use vegetables as the base and add just enough fruit for flavor. One apple or a handful of berries in a vegetable-heavy juice is a different situation than juicing four oranges and a mango.
Fruits That Simply Don’t Juice Well
Some produce isn’t dangerous, just impractical. Bananas don’t contain enough liquid to yield juice when pressed. You’ll get a paste that clogs your machine rather than anything drinkable. The same goes for avocados, which are too creamy and fatty to extract juice from. Mangoes fall into this category too, producing mush rather than juice. All three work well in a blender for smoothies but aren’t suited for a juicer.
Freshness Matters More Than You Think
Even safe, well-chosen ingredients lose value quickly once juiced. Vitamin C in juice degrades at about 2% per day once exposed to air. That means juice left in your fridge for a week has lost roughly 14% of its vitamin C content. The enzymes and other delicate nutrients break down on a similar timeline. If you’re going to the trouble of juicing, drink it the same day. Batch-prepping juice for the week undermines much of the nutritional point.