Avoid putting carbonated drinks, cream liqueurs, citrus-based cocktails, beer, wine, or hot milk in a stainless steel hip flask. These liquids can spoil quickly, cause pressure build-up, or react with the stainless steel, leading to unpleasant tastes or damage. Mixed drinks with soda or juice are also a bad fit, as they can leave behind residue that’s hard to clean. For the best experience, stick with hard liquors like whiskey, rum, or vodka, and always clean your metal hip flask properly after use.