Pilot Trial of Domperidone in Relapsing-Remitting Multiple Sclerosis (RRMS)

Inclusion Criteria: * Participants must be between age 18 and 60 years. * Sexually active men and women of child-bearing potential, defined as those who are not postmenopausal (24 consecutive months) or permanently sterilised, must agree to use adequate contraception. Adequate contraception is defined as methods of birth control which result in a low failure rate \[i.e. less than 1% per year\] when used consistently and correctly such as implants, injectables, combined oral contraceptives, some intrauterine devices (IUDs), barrier contraceptives, sexual abstinence or vasectomised partner. Adequate contraception is required during domperidone treatment and for one month after stopping treatment. * Participants must have MS defined according to the McDonald criteria (Polman et al. 2011). * Participants must have RRMS according to Lublin et al. (2014). * Participants must have been treated with a stable approved dose of glatiramer acetate, interferon-beta, fingolimod, dimethyl fumarate, or teriflunomide for at least 6 months. * Participants must be scheduled to have a clinically indicated brain MRI to monitor DMT effectiveness. * Participants must have at least one gadolinium enhancing lesion on their DMT monitoring MRI (screening MRI). * Participants must report at least 80% adherence to their current DMT and have had no treatment discontinuation for one week or longer within the prior 6 months. * Participants must be patients of the Calgary MS Clinic. * Participants must provide written informed consent Exclusion Criteria: * Participants who are pregnant or breastfeeding * Participants in whom it is expected that their current DMT will be discontinued within 16 weeks. Indicators of likely discontinuation will include poor DMT tolerance, a wish by the participant to discontinue the DMT, or a history of poor clinical response to the current DMT (such as a relapse within the previous year while on the current DMT) * Participants who have a long QT interval, defined as corrected QT interval of more than 470 msec in men and more than 450 msec in women, on screening ECG. * Participants with known long-QT syndrome or known cardiac arrhythmia. * Participants who currently or previously have been treated with natalizumab. * Participants who are currently treated with domperidone or have taken it within the previous 3 months. * Participants who are taking drugs which prolong QT intervals or who are treated with drugs that inhibit cytochrome P450 3A4. * Participants who are concurrently treated with drugs that may increase serum prolactin levels. * Participants who have a prolactinoma * Participants who use systemic corticosteroids within the 8 weeks prior to the screening MRI or who use concurrent immunosuppressive medications including systemic corticosteroids. * Participants who have a history of breast cancer or breast carcinoma in situ or who have clinically significant depression, renal, hepatic, cardiovascular, respiratory, metabolic, ophthalmologic, cerebrovascular, or other serious physical disease. * Participants in whom gastrointestinal stimulation might be dangerous, i.e. gastrointestinal hemorrhage or mechanical obstruction or perforation * Participants who have a known allergy or other intolerability to domperidone. * Participants with serum potassium, sodium, magnesium or calcium levels outside the normal range, serum creatinine \> 6 mg/100 ml or \> 0.6 mmol/l), or who have any other condition or situation that in the opinion of the investigator would either put the patient at risk of worsening health if enrolled in the trial or would prevent completion of the trial. * Participants who are concurrently participating in any therapeutic clinical trial.