The aim of this randomized, double-blind, placebo-controlled, phase II trial, is to study the effect of sonlicromanol on fatigue in patients with post-COVID who experience post-exertional malaise (PEM).

Inclusion Criteria:

• Post COVID according to WHO criteria and verified by post COVID physician
• Post-exertional malaise (PEM) according to DSQ-PEM questionnaire
• Bell's disability score 20-70%
• Mild initial SARS-CoV-2 infection (no hospitalisation)
• WHO performance score of 0 before initial SARS-CoV-2 infection

Exclusion Criteria:

• Patients at risk for cardiac conduction disorders
• History of clinical significant gastro-intestinal surgery or dysmotility that impairs the adsorption of the IMP
• Clinically significant respiratory or cardiovascular disease
• Unstable neurological disease
• Clinically significant active psychiatric disorder that requires treatment
• History of substance abuse
• Active malignancy within the past 5 years
• History of solid organ transplantation
• Active HIV, hepatitis B or C infection
• BMI < 18.5 or > 35
• Pregnancy or breastfeeding
• Clinically relevant laboratory test value outside the reference range
• Use of the following medication, unless stable for at least one month before study and remaining stable throughout the study: (multi)vitamins, co-enzyme Q10, Vitamin E, riboflavin, amino acids, antioxidant supplements and any medication negatively influencing mitochondrial functioning (including but not limited to valproic acid, glitazones, statins, antivirals, amiodarone and NSAIDs)
• Use of the following medication: any moderate or strong Cytochrome P450 (CYP)3A4 inhibitors (all 'conazoles-anti-fungals', HIV antivirals, grapefruit), strong CYP3A4 inducers ((including HIV antivirals, carbamazepine, phenobarbital, phenytoin, rifampicin, St. John's wort, pioglitazone, troglitazone) or any medication metabolized by CYP3A4 with a narrow therapeutic index, medication known to be substrate of Organic Cation Transporter 1 (OCT1) and organic cation transporter 2 (OCT2) or strong P-glycoprotein inhibitors (including amiodarone, azithromycin, captopril, clarithromycin, cyclosporine, piperine, quercetin, quinidine, quinine, reserpine, ritonavir, tariquidar, and verapamil).
• Use of any medication known to affect cardiac repolarization unless QTc interval at screening is normal during stable treatment for a period of two weeks, or 5 half-lives