A single remedy is a widely researched and studied remedy which before its use in the clinic has been extensively tested on healthy human volunteers (usually students of homeopathy and homeopathic physicians) for its effects. Single remedies used appropriately are 100% safe.
Multi-ingredient medicines (also called mixtures and combinations) are a concoction of two or more single remedies usually in a low strength – Ø, 1X, 2X, 3X, 3C, 6C, etc. The problem with multi-ingredient medicines is that they contain homoeopathic ingredients but the method of their preparation is not that of homoeopathic pharmacy. Further, there is no dependable scientific study to support the claims about the efficacy and safety of these medicines. While evidence suggests, they are not totally ineffective, they have all the potential to complicate a case by suppressing or masking original symptoms of the patient. If not anything else, their use can delay the proper medical care. Also see: Are multi-ingredient medicines (Combination Numbers) really effective?