The beverage comprises carbonated water, citric acid, sodium citrate (acidity regulator), natural flavourings including quinine, and sweetener (aspartame). These ingredients combine to deliver the signature taste of Schweppes Slimline Tonic Water, balancing sweetness with the characteristic bitterness of quinine.​

Schweppes Slimline Tonic Water is free from the 14 major allergens, including gluten, dairy, nuts, soy, and sulphites, making it suitable for individuals with common food allergies. It is also gluten-free, GMO-free, and suitable for vegetarians and vegans. However, it contains aspartame, a source of phenylalanine, which may be unsuitable for individuals with phenylketonuria (PKU). Consumers are advised to check the product label for the most accurate and up-to-date information.