Primidone has other cardiovascular effects in beyond shortening the QT interval. Both it and phenobarbital are associated with elevated serum levels (both fasting and six hours after methionine loading) of homocysteine, an amino acid derived from methionine. This is almost certainly related to the low folate levels reported in primidone users. Elevated levels of homocysteine have been linked to coronary heart disease. In 1985, both drugs were also reported to increase serum levels of high density lipoprotein (HDL) cholesterol, total cholesterol, and apolipoproteins A and B.