Inclusion Criteria:
For male and female participants:
* Ability to confirm voluntary participation and approve the Informed Consent Form;
* Men and women aged 18 to 60 years (inclusive);
* Body weight between 50-120 kg for men and 40-90 kg for women;
* BMI ≤34.9 kg/m²;
* Complete ACL rupture visualized by pre-operative magnetic resonance imaging (MRI);
* Having undergone arthroscopic knee surgery for anterior cruciate ligament (ACL) coverage using an autologous hamstring tendon graft;
* Presenting with an isolated ACL injury or combined with ligamentous, meniscal, or cartilage lesions visualized by MRI, provided they do not interfere with the rehabilitation protocol.
* Classification as very active, active, or irregularly active type A according to the International Physical Activity Questionnaire (IPAQ), based on pre-ACL injury physical activity;
Inclusion criteria assessed at the Randomization Visit (VR/V2):
* Continue to meet all inclusion criteria verified during the Selection Visit (VS/V1)
* Adherence to the rehabilitation protocol, having initiated postoperative physiotherapy treatment;
* Functional range of motion from 0 to 120º and ability to ambulate without crutches;
* Blood pressure in the seated position in the doctor's office \<180/95 mmHg;
* Hematocrit ≤ 50%;
* ALT less than three times the upper limit of normal;
* Serum creatinine \<2 mg/dL;
* Total bilirubin \< 3.0 mg/dL;
* Albumin ≥ 3.5 g/dL;
For male participants only:
\- Total PSA ≤ 4.1 ng/mL.
Exclusion criteria:
For female participants only:
* Confirmed or suspected pregnancy;
* History of childbirth, abortion, or lactation in the last 3 months;
* Refusal to use permitted contraceptive methods during the study and for 90 days after the end of participation in the study, unless surgically sterile or expressly declaring themselves exempt from the risk of pregnancy due to not engaging in sexual activity or engaging in non-reproductive activity;
* Clinical signs of hyperandrogenization characterized by: hirsutism defined by a Ferriman-Gallwey score ≥ 8; or alopecia defined by hair loss of at least 50% of the participant's normal hair, which is not obvious from a distance but is only noticeable upon closer inspection; a different haircut may be necessary to cover the hair loss, but does not necessarily require a wig or hairpiece to camouflage it; or Grade 5 acne defined by a predominance of inflammatory acne lesions in the facial area;
* Polycystic Ovary Syndrome;
* Known or suspected breast carcinoma;
For male participants only:
\- Known or suspected carcinoma of the prostate or male breast;
For male and female participants:
* Previous serious injury or history of surgery on the lower limbs;
* Knee injury more than 36 months ago;
* Meniscal tear requiring repair or suturing during ACL reconstruction surgery that interferes with postoperative rehabilitation (e.g., immobilization or limitation of range of motion);
* Use of patellar, quadriceps, or other hamstring tendon grafts during ACL reconstruction;
* Known contraindication to hormone use;
* Any condition that worsens under hormone treatment;
* Personal history of deep vein thrombosis (DVT);
* Known coagulopathy;
* Known chromosomal disorders;
* Hypersensitivity to anabolic androgenic steroids;
* Previous treatment failure with Oxandrolone;
* Concomitant use of testosterone (or analogues) and other anabolic androgenic steroids, or prior use without completion of an adequate washout period before baseline serum testosterone sampling for study eligibility determination and randomization, in any pharmaceutical formulation within the last 3 months. Practical clinical washout will be based on recovery of baseline serum testosterone levels to \<300 ng/dL. The minimum recommended washout periods are 2-3 days for transdermal gel formulations, 3 weeks for short-acting testosterone esters (enanthate or cypionate; intramuscular or subcutaneous), and 8 weeks for long-acting testosterone esters (undecanoate; intramuscular).
* Pituitary tumor;
* Creatinine levels \>2 mg/dL or history of chronic kidney disease;
* Myocardial infarction in the last 6 months;
* Uncontrolled dyslipidemia;
* Uncontrolled diabetes;
* Patients with chronic obstructive pulmonary disease (COPD) unresponsive to bronchodilators;
* Concomitant use of warfarin or another oral anticoagulant during experimental treatment.
* Irregularly active type B or sedentary classification on the International Physical Activity Questionnaire (IPAQ), based on pre-ACL injury physical activity;
* Athlete engaged in paid physical activity;
* Known psychiatric diagnosis, including disorder Major or persistent depressive disorder, bipolar disorder, anxiety, social phobia, specific phobias or obsessive-compulsive disorder, psychotic disorder, personality disorder, eating disorder, neurocognitive disorders, and developmental or somatoform disorders (somatization or hypochondria);
* Presence of voiding disorder;
* Known diagnosis of fibromyalgia;
* Participation in other clinical trial protocols in the last 30 days;
* Participant who, in the investigator's opinion, presents other conditions or clinical or laboratory alterations that make them ineligible to participate in the study;
Exclusion criteria assessed at the Randomization Visit (VR/V2):
* The participant came to meet any exclusion criterion verified at the Selection Visit (SV/V1);
* Presenting postoperative complications such as stiffness and/or healing in the knee requiring additional procedures.
