Now this patient begins to use cocaine which causes an increase in the number of opiate receptors to 150. However, only 75 receptors remain filled and active. Now instead of 75 percent of the receptors being filled now only 50 percent are filled. The patient complains that the cocaine is eating up their methadone and asks for a raise. Barbiturates There has been one or two reports of a barbiturate causing abstinence in a methadone patient.
Some drugs can interact with methadone when it is bound to plasma proteins.
As was mentioned above most binding of methadone to plasma proteins is non specific, and this means that many drugs with similar shapes can bind to the same area of the blood protein and knock the methadone molecule out. This can cause a much higher effective blood methadone level, which can be a problem for someone who does not have tolerance or even someone on a low dose of methadone without much of an initial tolerance. Some drugs can knock methadone away from the blood proteins that they are bound to causing a sudden release of methadone to them interact with the opiate receptor. It would probably take a few days to build up the methadone plasma to its initial level. It is possible that this effect could be used 'therapeutically' when higher methadone levels are desired, but it could also be very dangerous.