A Study of LY3873862 in Participants With Sporadic or Familial Amyotrophic Lateral Sclerosis (ALS).

Inclusion Criteria: * Have a diagnosis of sporadic or familial Amyotrophic Lateral Sclerosis (ALS) (definite, probable, laboratory probable) made by physician experienced with management of ALS as defined by the World Federation of Neurology revised El Escorial criteria. * Have an ALS symptom onset as determined by the investigator within 24 months of screening. * Have a body mass index within the range 18 to 35 kilograms per square meter (kg/m²) (inclusive). * Are individuals assigned male at birth (AMAB) or assigned female at birth (AFAB), including both individuals of childbearing potential (IOCBP) and individuals not of childbearing potential (INOCBP). Contraceptive use by participants should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. * Are able to safely swallow solids or liquids, or both. Exclusion Criteria: * Have a history or presence of medical illness that, in the judgment of the investigator, indicates a medical problem that would preclude study participation including, but not limited to cardiovascular, renal, hepatic, gastrointestinal, respiratory, hematological, endocrine, psychiatric, neurologic, convulsions, or any clinically significant laboratory abnormality. * Have a history of another neurodegenerative disease or significant dementia/severe cognitive problems at screening. * Have a slow vial capacity (SVC) less than (\<) 50% predicted value (for sex, age, ethnic origin, and height) at screening. * Have significant hepatic abnormalities (aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \>2 times (×) upper limit of normal (ULN) or total bilirubin (TBL) \>1.5× ULN, in asymptomatic individuals with a history of Gilbert's syndrome, use TBL of \>2× ULN). * Have a significant renal impairment (estimated glomerular filtration rate less than 60 milliliters per minute per 1.73 square meters (eGFR\<60 mL/min/1.73m²)). * If taking riluzole, must be on a stable dose for more than 30 days (prior to Day 1). * If taking intravenous or oral edaravone, must have completed 2 treatment cycles (prior to Day 1). * Are currently taking or plan to take tofersen for the duration of the study. * Show evidence of Human Immunodeficiency Virus (HIV) and/or positive human HIV antibodies. * Show evidence of hepatitis B and/or positive hepatitis B surface antigen. * Have a positive Hepatitis C Virus (HCV) antibody test. Participants with a positive HCV antibody test at screening can be included only if a confirmatory HCV Ribonucleic Acid (RNA) test is negative. * Have an abnormal blood pressure judged to be clinically significant by the investigator. * Have a history of clinically significant back pain, pathological conditions of the back, or back injury (for example, degenerative disease, spinal deformity, or spinal surgery), have a local infection at the intended site of the lumbar puncture (LP) that may predispose to complications or technical difficulty with LP. * Are taking or intend to take inhibitors or inducers of CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 from 14 days prior to the first dose of any study intervention, or for 5 half lives (whichever is longer), until the final PK sample is collected.