Inclusion Criteria:
* Women
* All pre-menopausal women aged above 18 years
* Diagnosed with endometriosis (ultrasound, MRI or previous laparoscopic and/or diagnostic surgery) according to the #Enzian classification \[52\]. This means that endometriosis is present in the following compartments:
* Rectovaginal space (minimal A1) and/or
* Sacrouterine ligaments, cardinal ligaments, pelvic sidewall (minimal B1) and/or
* Rectum (minimal C1) and/or
* Endometriosis of the intestines, diaphragm and/or
* Adenomyosis (according to the morphological uterus sonographic assessment (MUSA) or evident adenomyosis on the MRI) \[53, 54\] and/or
* Peritoneal / superficial endometriosis (diagnosed laparoscopically and not treated during surgery).
* Mild to severe chronic pelvic pain (NRS scale \>= 6). The 11-point NRS scale ranges from '0' representing no pain to '10' representing the worst pain imaginable.
* Resistant to current recommended lines of analgesics (paracetamol, NSAIDs)
* Usage of strong opioids must not have been prescribed or otherwise have been discontinued for more than 1 week.
* An indication for endometriosis resection surgery or on the waiting list for surgical treatment
* Ability to understand the patient information letter and to give oral and written informed consent
* No alteration in the utilization of hormonal therapy ≤1 months prior to inclusion.
Exclusion Criteria:
* Pain score \<6 out of 10 (NRS) for chronic pelvic pain
* Endometriosis affecting the bladder and ureter
* Increased intracranial pressure
* Poorly regulated hypertension, \>180/100mmHg at rest
* Patients with thyroid disease
* Patients with cancer
* History of psychiatric illness (schizophrenia, psychosis, delirium, manic depression)
* Serious medical disease (e.g., cardiovascular, renal , pulmonary or liver disease)
* Severe liver disease
* Patients with glaucoma
* Usage of strong opioid medication
* Usage of xanthine derivatives or ergometrine
* Unstable angina, heart failure, history of cerebral vascular accident (CVA)
* Patients suffering from an active infection
* Patients with epilepsy
* Patients trying to achieve pregnancy and or patients who are breastfeeding
* Not being able to answer questionnaires (in Dutch)
* Mentally incompetent (patients not able to make decisions that are in their best interests, this will be evaluated by their treating physician (e.g. patients with an intellectual disability or mental retardation))
* Alcohol or drug abuse
* Patient with a known (es)ketamine allergy
* Abnormal liver enzyme levels at baseline (ASAT, ALAT, GGT, AF, Bilirubin total)
Patients are allowed to continue the following pain medications: paracetamol, non-steroidal anti-inflammatory drugs as described previously by Sigtermans et al. (Trial NL466 (NTR507))\* according to their stable use in dose and frequency.
\*in case of tramadol, amitriptylin, selective serotonin reuptake-inhibitors, gabapentin and pregabalin, the usage may also be continued during this study.
