Metformin IR (immediate release) is available in 500 mg, 850 mg, and 1000 mg tablets.

Metformin SR (slow release) or XR (extended release) was introduced in 2004, in 500 mg and 750 mg strengths, mainly to counteract the most common gastrointestinal side effects, as well as to increase patient compliance by reducing pill burden. No difference in effectiveness exists between the two preparations.

Combinations

Metformin is often prescribed to type 2 diabetes patients in combination with rosiglitazone. This drug actively reduces insulin resistance, complementing the action of the metformin. In 2002, the two drugs were combined into a single product, Avandamet, marketed by GlaxoSmithKline. In 2005, all current stock of Avandamet was seized by the FDA and removed from the market, after inspections showed the factory where it was produced was violating Good Manufacturing Practices. The drug pair continued to be prescribed separately in the absence of Avandamet, which was available again by the end of that year.

In the United States, metformin is also available in combination with pioglitazone (trade name Actoplus Met), the sulfonylureas glipizide (trade name Metaglip) and glibenclamide (trade name Glucovance), and the dipeptidyl peptidase-4 inhibitor sitagliptin (trade name Janumet).