Dec 20 (Reuters) - The U.S. Food and Drug Administration (FDA) on Monday approved an injectable form of GlaxoSmithKline Plc"s (GSK.L) cabotegravir drug that is given every two months to prevent an HIV infection. The injection - Apretude - to prevent the virus that causes AIDS is aimed as an alternative to daily pills and has been approved for use in at-risk adults and adolescents. Its first two doses are administered one month apart and then the injection is given every two months thereafter. Patients can start their treatment with Apretude or take oral cabotegravir for four weeks to assess how well they tolerate the drug. Apretude comes with a boxed warning to not use the drug unless a negative HIV test is confirmed. A boxed warning on the label is FDA’s strictest warning and calls attention to serious or life-threatening risks of a drug. Apretude"s approval was based on a final study showing it was found to be 66% more effective in preventing HIV infections than Gilead"s (GILD.O) Truvada daily oral pills. Cabotegravir and the daily oral pill were both well tolerated in the study, which was conducted on men who engage in sexual activity with other men and transgender women who have sex with men, putting them at risk of contracting HIV.