A class of diabetes drugs that include Merck's Januvia have been linked with severe joint pain, the U.S. Food and Drug Administration said on Friday.

The FDA said it had identified 33 cases of severe joint pain in patients taking a class of drugs known as DPP-4 inhibitors between Oct. 16, 2006, when the first one was approved, through Dec. 31, 2013.

The most frequent number of cases, 28, occurred with Januvia, known generically as sitagliptin.

Five cases were reported with AstraZeneca's Onglyza (saxagliptin) two with Boehringer Ingelheim's Tradjenta (linagliptin) and one with Takeda Pharmaceutical's Nesina (alogliptin).

The FDA said that in 20 of the cases the DPP-4 inhibitor was suspected as a cause of the pain and was discontinued within a month after the onset of symptoms.

In eight of the remaining 13 cases, a period of 44 days to a year elapsed between the onset of symptoms and discontinuation of the drug.

The drugs are used to lower blood sugar in adults with type 2 diabetes.

The FDA said patients should not stop taking their drugs but should contact their doctor if they experience severe and persistent joint pain.