St. John’s wort, similarly to steroid hormones and some phytotherapeutic drugs, may increase hepatic levels of CYP3A4, accelerating the metabolism of some drugs. Also, several foods contain inhibitors or inducers of CYP, thus influencing the metabolism and toxicity of drugs. Grapefruit juice is a strong inhibitor of CYP3A4, and may increase the bioavailability of drugs metabolized by CYP3A4. Individual differences in CYP polymorphisms have an influence on drug metabolism [5, 6, 105].

All nutritional supplements should be reviewed when the patient is undergoing cancer treatment, because many foods and supplements may alter the therapeutic efficacy and toxicity of chemotherapeutic drugs.

St. John’s wort induces CYP3A4, 2D6, 2C9, and 1A2, and so may increase the clearance of chemotherapy drugs metabolized by these cytochrome enzymes, reducing these drugs’ effects [23, 105].

Multivitamins containing folic acid may reduce the therapeutic efficacy of methotrexate, taking into account the mechanism of action of methotrexate – a structural analogue of folic acid – which is a folic acid inhibitor [23].

Mercaptopurine oral may be best absorbed if taken 1 h before or 2 h after the last meal, although dairy or calcium products should be avoided within 2 h of the dose [67, 104]. Mercaptopurine is inactivated by xantine oxidase; therefore, this substance may potentially reduce the bioavailability of mercaptopurine. Cow’s milk contains a high level of xanthine oxidase and should be avoided with mercaptopurine. This interaction may be clinically significant [68].

For temozolomide oral, the absorption of the medication can be reduced if it is taken with food, thus the increasing its adverse effects [67].

Melphalan oral has a reduced effect if taken with food [67].