(metoprolol tartrate) Tablets, USP. (metoprolol tartrate) Injection, USP. Lopressor, metoprolol tartrate USP, is a selective betai-adrenoreceptor blocking agent, available as 50- and 100-mg tablets for oral administration and in 5-mL ampuls for intravenous administration. Each ampul contains a sterile solution of metoprolol tartrate USP, 5 mg, and sodium chloride USP, 45 mg, and water for injection USP.