Compositions and methods are provided for inhibiting a polymerase from replicating non target DNA at a temperature below the amplification reaction temperature. The inhibitor is a synthetic nucleic acid which is single stranded but folds to form at least one double stranded region designed to melt at a temperature which is lower than the amplification reaction temperature, and at least one single stranded region where the single stranded region at the 5' end contains at least one uracil or inosine and optionally a sequence at the 3' end contains one or more derivative nucleotide or linkages.