It is readily absorbed following oral administration. The drug is biotransformed in the liver and is excreted almost exclusively in feces. It is important to understand the pharmacokinetics of methadone when using this medication, due to multiple variables associated with it. Methadone is very lipophilic, leading to accumulation in the fat tissues. The slow release from these fat tissues causes the half-life to range from 12 to 40 hours and has been reported to extend up to 150 hours. The actual duration of analgesia ranges from 4 to 8 hours. Upon repetitive dosing, methadone levels can accumulate due to this long terminal half-life, thereby leading to toxicity. The metabolism is variable because it relies on multiple cytochrome P450 (CYP450) enzymes, some of which are affected by known genetic polymorphisms and are susceptible to many drug interactions.