The drug major Merck has launched the first two head-to-head clinical trials that compare the efficacy and tolerability of Fosamax (alendronate) once weekly 70mg and Actonel (risedronate) once-weekly 35mg. Fosamax and Actonel, both bisphosphonates indicated for the treatment of osteoporosis in post menopausal women are co-marketed by Aventis and Procter & Gamble through the Alliance for Better Bone Health. The trial called FACTs(Fosamax Actonel Comparison Trials), is being sponsored by Merck as a US-based study, as well as an international study, each of which will examine approximately 800 post menopausal women with osteoporosis(defined by a bone mineral density (BMD) T-score of <=-2). The participants will be randomized in a one-to-one blinded fashion to either alendronate 70mg once weekly or risedronate 35mg once weekly. The trials will then evaluate and compare the effectiveness and tolerability of the 2 agents during 12 months of treatment in the post menopausal women with osteoporosis.