The causes of low blood calcium include insufficient intake, disease factors (e.g. hypoparathyroidism, etc.) and drug factors (e.g. rifampicin, etc.). 1. Inadequate intake: Long-term malnutrition may lead to insufficient calcium intake and low blood calcium. In addition, vitamin D can promote calcium absorption, and insufficient vitamin D may also lead to low blood calcium. 2. Disease factors: hypoparathyroidism can cause low blood calcium, high blood phosphorus, abnormal loss of calcium caused by renal disease can lead to low blood calcium, diarrhea, constipation impede the absorption of calcium, can also lead to low calcium. 3. Drug factors: taking certain drugs such as phenytoin sodium, rifampicin, etc. can increase the metabolism of vitamin D, which can lead to low calcium. In addition, there may be other reasons for low calcium, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, and then give targeted treatment under the guidance of the doctor.