Short answer: Yes, but you should be very careful with dosages and the context you take them in.
Obviously methadone is a very potent and long lasting narcotic analgesic, high doses of methadone alone can cause somnolence (sleepiness).
Quetiapine is a potent antipsychotic with very strong sedative properties. It is used to treat people diagnosed with schizophrenia to limit psychotic symptoms, and those with bi-polar to prevent manic episodes. It is also commonly used as an anti-anxiety medication and sleep aid in patients where sedatives of the benzodiazepine, z-drug, or barbituarate classes are contraindicated due to addiction concerns.
Combined the effects of both drugs are significantly amplified and can cause profound somnolence. It is not advisable, unless directed by a doctor, to consume the drugs in combination during the day; you should not drive while under their influence or operate heavy machinery.
I am a methadone patient (90mg daily) and am prescribed quetiapine (25mg tablets) to help with sleep when required. I only take them occasionally, and I only take half at a time. That is still enough to knock me out - equally as or even more effectively as the equivalent dose of Klonopin (a benzo) - and leaves me feeling a little groggy on waking.
Having said that I know people who are prescribed them to control mania, and the dosage range is far greater. One guy I know takes 300mg in the morning with breakfast and then another 500mg at night, so tolerance does build. The point is be careful, sensible and seek advice from a doctor.
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