Skin and Soft Tissue Infections (SSTIs) due to IV drug use: This is the most likely diagnosis given the patient's history of IV drug use and the presence of open, oozing abscesses on the forearms. IV drug users are at high risk for developing SSTIs, particularly those caused by Staphylococcus aureus and Streptococcus pyogenes.

Cellulitis: The smaller, erythematous papules on the arms and legs could be indicative of cellulitis, a common infection of the skin and subcutaneous tissue.
Folliculitis: The presence of smaller papules on an erythematous base could also suggest folliculitis, an inflammation of the hair follicles, which can be caused by bacterial infection, often related to poor hygiene or contaminated equipment in IV drug users.
Pyoderma Gangrenosum: Although less common, pyoderma gangrenosum could be considered, especially if the lesions are extremely painful and have a characteristic appearance of a deep ulcer with undermined edges.

Necrotizing Fasciitis: This is a life-threatening condition that requires immediate attention. Although it may not be as common as other diagnoses, the presence of severe pain, swelling, and skin necrosis, especially in the context of IV drug use, necessitates consideration of necrotizing fasciitis.
Endocarditis: IV drug users are at increased risk for infective endocarditis, and while skin lesions alone might not directly suggest this diagnosis, the overall clinical context (e.g., fever, heart murmur) could warrant consideration to avoid missing a potentially fatal condition.
Osteomyelitis or Septic Arthritis: These conditions can occur in IV drug users, especially if they have injected drugs into joints or bones, leading to localized infections that can be severe and require prompt treatment.

Lymphogranuloma Venereum (LGV): Although more commonly associated with genital lesions, LGV can cause skin lesions and should be considered in the differential diagnosis of a patient with a history of high-risk sexual behavior or exposure to LGV.
Cutaneous Leishmaniasis: If the patient has a history of travel to endemic areas, cutaneous leishmaniasis could be a rare but possible diagnosis, presenting with skin lesions that can resemble abscesses or ulcers.
Ecthyma Gangrenosum: Typically associated with Pseudomonas aeruginosa bacteremia, this condition presents with characteristic skin lesions and is more common in immunocompromised patients, but could be considered in an IV drug user with signs of sepsis.