A dietary supplement, also known as food supplement or nutritional supplement, is a preparation intended to supply nutrients, such as vitamins, minerals, fatty acids or amino acids, that are missing or are not consumed in sufficient quantity in a person's diet. Some countries define dietary supplements as foods, while in others they are defined as drugs. It is one of the most commonly practiced anti-ageing methods.

Most people consider that a dietary supplement is a substance which may be taken as a supplement but which is also contained in a normal diet. Other substances sold as dietary supplements include hormones and herbal medicines. Various hormones which have no prescribed application in mainstream medicine may be regarded as dietary supplements in some countries although they may be restricted in other countries. Herbs too are regarded as dietary supplements although a clear distinction needs to be made between non-toxic nutritive herbs and the more toxic and pharmacologically active herbs. Dietary supplement is a common yet powerful anti-ageing method, if applied properly.