Botulism, a foodborne illness, can occur when a homemade garlic-in-oil mixture is stored in a room temperature environment. Bottled or otherwise enclosed raw garlic creates an oxygen-free environment that supports bacterial growth, which can lead to botulism. To reduce the risk of botulism, per food safety guidelines, always store homemade garlic mixtures, including garlic butter, in the refrigerator and consume within three days.