People with allergies to kava or kavapyrones should not take kava. Skin rashes have been reported after taking kava.

Until recently, kava was generally thought to be safe: when used in otherwise healthy people not taking any other drugs, herbs, or supplements; over short periods of time (one to two months); and at recommended doses. However, there have been numerous reports of severe liver problems in people using kava. Multiple cases of liver toxicity, including liver failure, have been reported following the use of kava in Europe. The U.S. Food and Drug Administration (FDA) has issued warnings to consumers and physicians and has requested that physicians report cases of liver toxicity that may be related to kava use. Although many natural medicine experts still believe that kava is safe at recommended doses, there is not enough scientific information to make a clear conclusion. Therefore, kava should be used only under the supervision of a qualified healthcare professional, should never be used above recommended doses, and should be avoided by people with liver problems or taking drugs that affect the liver.

Other serious side effects that have been observed with chronic or heavy use of kava include: skin disorders, blood abnormalities, apathy, kidney damage, seizures, psychotic syndromes, and increased blood pressure in the lungs (pulmonary hypertension). Blood in the urine has also been reported.

Mild side effects may include gastrointestinal (stomach) upset, allergic rash, or mild headache.

Several cases of abnormal muscle movements have been reported after short-term use of kava (one to four days), including tightening, twisting, or locking of the muscles of the mouth, neck (torticollis), and eyes (oculogyric crisis). Worsening of symptoms of Parkinson's disease and several cases of abnormal whole body movements (choreoathetosis) following high doses of kava have also been noted. Tremor, poor coordination, headache, drowsiness, and fatigue have uncommonly been reported, particularly with large doses. A case of muscle cell breakdown (rhabdomyolysis) was reported in a 29 year-old man after taking an herbal combination of ginkgo, guarana, and kava.

Sedation (drowsiness) has occasionally been reported with kava use, although there is early evidence from several small human studies that kava may not significantly cause this effect. Because this issue remains unclear, driving and operating heavy machinery is not recommend while taking kava.

Eye disturbances and eye irritation have rarely been associated with chronic or heavy kava use. Rapid heart rate, electrocardiogram (ECG) abnormalities, and shortness of breath have been reported in heavy kava users. Laboratory tests suggest that kava may increase the risk of bleeding through effects on blood platelets.

Kava may affect electroconvulsive therapy (ECT) outcome. It has also been associated with meningismus (pain caused by irritation in the layers around the brain and spinal cord), urinary retention, skin lesions, enhanced or decreased cognitive performance, anorexia, sleeplessness, abnormal sensations called paresthesias, vomiting, and dangerously high blood pressure.

Use of kava cannot be recommended during pregnancy or breastfeeding. There may be decreases in the muscle strength of the uterus with the use of kava, which may have harmful effects on pregnancy. Chemicals in kava may pass into breast milk with unknown effects, and therefore this herb should be avoided during breastfeeding.

Many doctors recommend starting with a low dose and gradually increasing intake over time. Typical doses range from 50 to 280 milligrams of kava lactones per day at bedtime. Sixty to 120 milligrams of kavapyrones have been taken daily. A dose of 50 to 100 milligrams taken by mouth has been used for up to two months. A dose of 100 milligrams of kava extract (WS 1490) has been taken three times daily. Doses as high as 800 milligrams daily of kava extract have been taken for short periods, but have not been studied over the long term and safety is not clear.

There is not enough scientific evidence to recommend the use of kava in children.

Based on multiple human reports of liver toxicity, including hepatitis, cirrhosis, and liver failure, a theoretical increased risk of liver damage may occur if kava is taken with drugs that may injure the liver such as alcohol or acetaminophen (Tylenol®). Chronic use of kava may lead to kidney damage. Agents broken down by the kidneys should be used cautiously with kava due to increased risk of kidney damage.

In theory, kava may increase the effects of alcohol or other drugs that cause sedation (drowsiness). In theory, kava may interfere with the effects of dopamine or drugs that are similar to dopamine and may worsen the neurologic side effects of drugs that block dopamine such as haloperidol (Haldol®).

Kava may have chemical properties similar to monoamine oxidase inhibitors (MAO-I). In theory, kava may add to the effects of MAO-I antidepressants, such as isocarboxazid (Marplan®), phenelzine (Nardil®), or tranylcypromine (Parnate®). Due to this possible effect, kava may also cause the effects of anesthesia to last longer and some practitioners recommend stopping kava two to three weeks before surgery.

Laboratory tests suggest that kava may increase the risk of bleeding through effects on blood platelets. However, human evidence is lacking in this area. People using aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) and heparin, or anti-platelet drugs such as clopidogrel (Plavix®) should be aware of possible interactions.

Since kava has diuretic properties, it may have additive effects when taken with diuretic drugs such as furosemide or with ACE-inhibitors such as benazepril or captopril. Avoid in Parkinson's disease or in patients with a history of medication-induced extrapyramidal effects because kava may cause additive effects. Kava may cause excessive drowsiness when taken with SSRI antidepressant drugs such as fluoxetine or sertraline. Buspirone and opipramol may have additive effects when taken with kava.

Early evidence shows that kava may interfere with the way the body processes certain drugs using the liver's "cytochrome P450" enzyme system. As a result, the levels of these drugs may be altered in the blood, which may cause different effects or potentially serious adverse reactions.

Kava may have additive sedative effects when taken concomitantly with the opioid analgesics oxycodone and propoxyphene.

Kava may also interact with anti-cancer drugs or hormonal drugs, such as birth control pills.

Based on multiple human reports of liver toxicity, including hepatitis, cirrhosis, and liver failure, a theoretical increased risk of liver damage may occur if kava is taken with herbs or supplements that may damage the liver.

Kava may increase the amount of sedation (drowsiness) caused by some herbs or supplements, such as valerian. Caution is advised while driving or operating machinery.

In theory, kava may add to the effects of herbs and supplements that act like monoamine oxidase inhibitor (MAO-I) drugs, such as evening primrose oil. It may also add to the effects of herbs that have activity similar to the class of antidepressants known as selective serotonin reuptake inhibitors (SSRIs).

Based on laboratory tests, it is suggested that kava may increase the risk of bleeding through effects on blood platelets. However, human evidence is lacking in this area. People using other herbs or supplements that may increase the risk of bleeding should speak with a healthcare professional before starting kava.

Since kava has diuretic properties, it may have additive effects when taken with diuretic herbs or supplements like horsetail or licorice.

Use cautiously with herbs or supplements that are broken down by the kidneys because kidney damage may occur.

Preliminary evidence shows that kava may interfere with the way the body processes certain herbs or supplements using the liver's "cytochrome P450" enzyme system. As a result, the levels of other herbs or supplements may be too high in the blood.

Kava may interact with herbs and supplements with anti-cancer or hormonal activity; use cautiously.