If you are taking methadone what does this mean for you.? If you live in the US, and probably most of the world (there may be places where you can get enantiomerically pure (R)-methadone, I just am not aware of any) you are getting a racemic (50:50) mixture of R/S Methadone. (R)-methadone is 10-50 times stronger than (S)-methadone, so for all practical purposes only half of your dose is actually an opioid.
This becomes relevant clinically because the different enantiomers have different pharmacological properties. Take peak-and-trough testing for example. Let's say you are on a high dose of methadone but wake up every morning feeling sick. You request an increase but since you are already on a high dose the clinic says, "Hold up there we can't take your word for it, you're a junkie and everyone knows junkies lie so we are going to confirm your feelings with a blood test."
Peak and trough testing is a blood test that measures the level of methadone at two points in a 24 hour period. The first is right before dosing when the methadone is lowest (the trough), then another blood test occurs about 3 hours after dosing when the methadone has reached maximum blood concentration (the peak). Supposedly based on this test they can tell if you really need additional methadone.
But here's the thing, the blood test does not distinguish between (S)- or (R)-methadone. While they are similar in structure and share many pharmacological properties, they are not the same. An individual may metabolize (R)-methadone faster than (S)-methadone, so that by the time they wake up in the morning the level of (R)-methadone is way down and they are beginning to feel withdrawal. However their blood is still loaded with (S)-methadone, so the peak and trough test, which does not distinguish between R and S enantiomers, gives results that indicate the level of methadone is adequate. And so the doctor with their test results can tell the patient it's all in their head, when in reality there is a clinical reason why the person wakes up every morning feeling ill. The patient should always be trusted when it comes to their own body.
There may be other differences in the two enantiomers. Methadone has action on other receptors besides the endorphin receptors. Would there be any advantages to using pure (R)-methadone? At this point I am not sure but will revisit this issue as I learn more.
Read more about Peak and Trough testing here.