Acute respiratory distress syndrome (ARDS) due to methadone (MTD) toxicity is a known but rather uncommon phenomenon. In most of the previously reported cases of MTD-related ARDS, MTD was ingested orally in the form of tablets in high or unknown amounts. Despite the findings from the available literature, this case report is aimed at demonstrating that even small amounts of MTD syrup can cause ARDS earlier than it is usually expected. We present a non-addicted MTD-overdosed patient who developed ARDS after ingesting a very small amount of MTD syrup. We suggest close monitoring of MTD-overdosed patients from at least 48 h to 72 h for possible respiratory complications such as pulmonary oedema.