Table 2 The unadjusted associations between serum methadone concentrations and the study variables in 83 participants. All the primary and secondary outcome variables were inserted one by one in linear mixed model

Total SOWS^b score

− 3.9

− 7, − 9.1

0.011

 Anxiety

− 4.3

− 7.6, − 1.1

0.009

 Yawning

− 0.5

− 3.5, 2.5

0.739

 Perspiring

− 0.4

− 4.1, 3.3

0.821

 Tearing

− 1.5

− 4.1, 1.2

0.283

 Nose run

− 1.7

− 4.9, 1.5

0.305

 Goose bumps

− 2.1

− 5.5, 1.4

0.244

 Shaking

− 2.7

− 5.7, 0.3

0.081

 Hot flushes

− 2.6

− 6.2, 8.9

0.143

 Cold flushes

− 3.2

− 7.1, 0.8

0.115

 Bone- and muscle ache

− 4.9

− 8.4, − 1.4

0.007

 Restlessness

− 5.2

− 9.6, − 0.8

0.021

 Nausea

− 1.5

− 4.2, 1.2

0.272

 Vomiting^d

–

–

–

 Muscle twitches

− 0.9

− 4.2, 2.4

0.590

 Stomach cramps

− 1.5

− 4.2, 1.1

0.263

 Feel like using^d

–

–

–

Subjective adverse effects^d

–

–

–

 Euphoria^d

–

–

–

 Perspiring (as adverse effect)

1.4

− 3.0, 5.9

0.526

 Nausea (as adverse effect)

3.1

0.1, 6.1

0.040

 Concentration difficulties

− 0.7

− 4.7, 3.3

0.731

 Feeling “brain fog”

1.5

− 2.2, 5.2

0.427

 Decreased sexual desire

2.2

− 2.4, 6.8

0.351

Self-reported substance use ^c

0.2

− 0.9, 1.3

0.754

 Heroin

− 2

− 3.8, − 0.2

0.028

 Other opioids^d

–

–

–

 Benzodiazepines

0.6

− 3.7, 4.9

0.772

 Cannabis

4.7

− 0.5, 9.9

0.077

 Amphetamines

− 0

− 3.0, 2.9

0.984

 Alcohol

− 4.5

− 7.8, − 1.2

0.008