Matrix metalloproteinases are involved in the pathogenesis of tendinopathy. Doxycycline, a widely available pharmaceutical agent mostly used for its antibiotic properties, also functions as an inhibitor of MMPs.

This study aims to investigate the effect of doxycycline treatment on lateral epicondylalgia (tennis elbow). During three weeks, patients receive doxycycline tablets 100 mg twice daily, or placebo. Main outcome variable is pain (VAS) at three weeks.

Serum and/or plasma levels of matrix metalloproteinases and tissue inhibitors of metalloproteinases are measured.

Inclusion Criteria:

• Age 18-65
• Pain at the lateral side of the elbow ≥8 weeks
• Local tenderness on palpation of the lateral epicondyle
• Pain on resisted extension of the wrist

Exclusion Criteria:

• Rheumatic disorder
• History of fibromyalgia or generalised pain
• Elbow surgery (on the painful side)
• Active infection
• Glucocorticoid treatment during the duration of symtoms of epicondylalgia
• NSAID use within 1 w before presentation and during the study
• Pregnancy
• Taking drug that might interact with doxycycline