Side effects of the donation of platelets generally fall into three categories: blood pressure changes, problems with vein access, and effects of the anticoagulant on the donor's calcium level. Blood pressure changes can sometimes cause nausea, fatigue, and dizziness. Venous access problems can cause bruising, referred to as a hematoma. While donating, a supply of calcium antacid tablets is usually kept close by to replenish the calcium lost. Because the anticoagulant works by binding to the calcium in the blood, a donor's levels of calcium – and especially of active calcium ions – drop during the donation process. The lips may begin to tingle or there may be a metallic taste; since calcium enables the function of the nervous system, nerve-ending-dense areas (such as the lips) are susceptible, at least during the donation process. Unusually low calcium can cause more serious problems such as fainting, nerve irritation and short-duration tetany. Such an acute hypocalcaemia is usually due to low calcium levels prior to donation, aggravated by the anticoagulant. Hypocalcaemia can be curtailed by modestly increasing dietary calcium intake in the days prior to donation. Serious problems are extremely rare, but apheresis donors are typically not allowed to sleep during the long donation process so that they can be monitored.