There are many reasons for a patient to consider the use of complementary therapies while undergoing treatment for cancer. The most common reasons relate to the complications associated with chemotherapy and radiation treatment, such as nausea and vomiting, neuropathy, diarrhea, and oral mucositis. Other common reasons for the use of alternative therapies relate to the complications associated with cancer itself, including cachexia, fatigue, pain, and poor quality of life.

The essential oil of peppermint (Mentha piperita) has also been widely used—and widely studied—as aromatherapy. Unlike lavender, which has most often been studied for the management of various forms of psychological distress, peppermint has received attention in relation to the management of gastrointestinal (GI) concerns. So far, clinical research suggests that it may be beneficial for chemotherapy-induced nausea and vomiting (CINV).

Honey has shown promise in the prevention and management of oral mucositis. Although not all studies agree, many do suggest that rinsing the mouth with honey or applying honey to oral lesions can reduce the severity of oral mucositis. Most of these studies have asked patients to rinse with, and then slowly swallow, about 20 mL of honey at least three times daily [10,11]. As a result of these findings, clinical practice guidelines from the Multinational Association of Supportive Care in Cancer and International Society of Oral Oncology (MASCC/ISOO) suggest the use of honey for the prevention of oral mucositis in patients with head and neck cancer who are receiving radiation. No recommendation is made for patients with other forms of cancer [12].

Propolis is a resinous material used by honeybees to construct and maintain the hive. It is typically derived from the buds of poplar or conifer trees and is purported to have antimicrobial and anti-inflammatory activity.

Similarly to honey, propolis has become a popular alternative therapy for oral mucositis, a use that is supported by the available research. Clinical studies have found that using a mouthwash containing propolis can reduce the severity of oral mucositis when compared with placebo. In most cases, the mouthwash was used to rinse the mouth for about one minute, three times daily [13,14,15].

There has also been interest in using propolis to improve quality of life and nutritional status during cancer treatment, although this has not been adequately assessed in clinical research.

Propolis is generally safe for most people to use either orally or topically. However, people with atopic disease may be more likely to experience an allergic reaction to propolis and should use these products with caution. In some cases, patients have experienced allergic contact dermatitis, including cheilitis, when propolis is used on or near the lips or mouth [16].

Two prescription drugs contain synthetic forms of delta-9-tetrahydrocannabinol (THC), the primary psychoactive ingredient found in cannabis. These prescription drugs—dronabinol (Marinol) and nabilone (Cesamet)—are approved by the U.S. Food and Drug Administration (FDA) for the treatment of CINV. Dronabinol is also approved for the management of weight loss and poor appetite (cachexia) in people with AIDS [23,24].

One of the most common sources of contamination in supplement cannabinoid products is the presence of a cannabinoid that is not listed on the label. For example, many CBD products have been found to contain large quantities of THC, despite the label claiming that the product contains pure CBD. Considering the psychoactive effects of THC, the presence of this chemical is often not desired by those who are purchasing a CBD product.

THC has been associated with increased heart rate, decreased blood pressure, dizziness, and fainting. Additionally, its psychoactive nature can produce mood changes, a sense of being "high," sedation, disorientation, paranoia, and anxiety. People who use THC should be cautious about operating a vehicle or heavy machinery, as it is also known to impair and slow a person's motor function and reaction time [28].

Many patients will choose to use cannabis and/or cannabinoids in conjunction with other treatment options, such as antiemetics and analgesics. In these cases, it is important to be aware of potential drug interactions. THC (as well as very high doses of CBD) produces CNS depressant activity, which can be compounded when used alongside other CNS depressants, such as opioids or benzodiazepines. Additionally, both THC and CBD have shown the potential to alter the activity of many enzymes in the cytochrome P450 (CYP) family, which are responsible for the metabolism of many prescription drugs. There is also some concern that different constituents in cannabis can alter platelet aggregation activity, which could increase bleeding risk [28].

Research on the use of L-carnitine for cancer-related cachexia, on the other hand, is more promising. Early studies have found that taking L-carnitine 4 grams daily for 12 weeks increases body mass index when compared to placebo [32]. Other research has found that when used in combination with megestrol acetate, L-carnitine improves lean body mass, fatigue, and quality of life when compared with megestrol acetate alone [33]. In the context of a benign safety profile and minimal adverse effects, L-carnitine can be considered as an adjunctive treatment for some patients with cancer-related cachexia.

A range of clinical studies show that taking high-dose melatonin (10–40 mg daily) in combination with conventional chemotherapy might improve tumor regression rate in certain types of cancer. Additionally, high-dose melatonin may help to reduce a range of chemotherapy-related complications, including hematologic complications, cachexia, asthenia, and neuropathy [35,36,37]. While these findings appear promising, patients should discuss the use of high-dose melatonin with their oncology specialists prior to use. While low doses of melatonin are relatively benign and carry few side effects, high doses of melatonin have not been studied as extensively. Some research suggests that high doses may stimulate immune function, increase sedation, increase bleeding risk, and lower the seizure threshold.

Astragalus (Astragalus membranaceus) is another commonly touted adaptogen. Unlike ashwagandha, however, astragalus has been studied for a variety of chemotherapy-related adverse effects, including nausea and vomiting, fatigue, and diarrhea. Unfortunately, these studies are relatively small in size and have not always employed a placebo control group. Additionally, most of this research has evaluated intravenous astragalus, which is not available in the United States [40,41,42]. Research on the use of oral astragalus is far more limited and preliminary.

The best evidence to date indicates that ginger is unlikely to be beneficial for the management of CINV. Thus, it may be appropriate to counsel patients against the use of this product. Although it is likely safe for most adults, patients taking anticoagulants or who are at an increased risk for bleeding should use ginger with caution. Ginger may reduce platelet aggregation and increase the risk for bleeding. While this concern is based primarily on laboratory research and has not been validated in clinical research, this risk should be weighed against the apparent lack of benefit [47].

Turmeric is generally safe when taken by mouth. The most commonly reported adverse effects are constipation, dyspepsia, diarrhea, and reflux. While rare, turmeric has been associated with reports of serious liver damage. It is not entirely clear whether turmeric was the cause of liver damage in these patients; however, in most cases, the damage resolved after discontinuation of the supplement. Turmeric should be used with caution in people with existing liver dysfunction or in those taking hepatotoxic drugs [52].

Vitamin E supplements have also been evaluated for CIPN, with the highest quality research showing no benefit [61]. As a result, ASCO does not recommend the use of vitamin E for the prevention or management of CIPN [62]. Similarly, research on the use of oral vitamin E for the management of breast cancer-related hot flashes found that it did not provide any benefits over placebo [63].

Many experts recommend caution with the use of vitamin E during cancer due to research indicating that vitamin E supplements may increase a person's risk for initially developing cancer, especially prostate cancer [64]. Additionally, some observational research suggests that people with head and neck cancer who take a vitamin E supplement during and after radiation therapy may have an increased risk of tumor recurrence or a second primary tumor [65]. Because there is currently no evidence of benefit with the use of vitamin E during cancer, consider steering patients away from these supplements.

When used short-term, zinc is considered generally safe, with only limited GI adverse effects. Due to this safety profile, zinc can be considered as a short-term adjunct treatment option for patients with oral mucositis. Doses used in studies of oral mucositis have ranged widely; those that have shown benefit utilized higher doses, providing zinc sulfate 220 mg three times daily [66,67]. Studies of mouthwashes that have demonstrated benefit contained zinc chloride 0.2% used twice daily, and zinc sulfate 1% used three times daily [68,69].

When widening focus to all patients with cancer, yoga continues to demonstrate benefit. Clinical studies show that yoga can reduce cancer-related fatigue. In response to this research, the National Comprehensive Cancer Network (NCCN) guidelines list yoga as a potential nonpharmacologic option for the management of fatigue in patients with cancer [73]. The benefits of yoga for chemotherapy-related fatigue, on the other hand, may be less profound and more short-term [72].

Although most research to date has been relatively low in quality, the available evidence suggests that acupuncture may be beneficial for cancer-related fatigue and pain. This has led the NCCN and ASCO to recommend the use of acupuncture for these purposes [73,76,77].

Small clinical studies indicate that acupressure may be beneficial for the management of cancer-related fatigue. It has also shown promise in the management of cancer-related pain. The NCCN and ASCO guidelines recommend acupressure as a nonpharmacologic treatment option for cancer-related pain [76,77].

Massage is another common modality that is used for the treatment of cancer-related complications. A large body of clinical research shows that massage therapy reduces pain and anxiety in patients with cancer when compared with no treatment or conventional care [86,87,88]. Additional clinical research shows that massage can improve sleep quality, fatigue, nausea, and mood in this population [89,90].

Based on the available evidence, massage can be recommended as an adjunctive treatment option for patients with cancer. Patients should request that the practitioner avoid any sensitive or at-risk areas during manipulation, particularly those who have recently had surgery or radiation.

To date, research suggests that participating in MBSR can improve cancer-related fatigue when compared with no intervention or a control group. However, most of this research suggests benefits may be short-term [91,92,93]. The NCCN guidelines list mindfulness as a nonpharmacologic option for patients undergoing cancer treatment or those who have completed treatment [73]. ASCO recommends this modality for patients who have finished cancer treatment [76].