Dec 15 (Reuters) - Bayer's eye medication Eylea does not offer an advantage over a rival product from Novartis when treating patients with a type of diabetes-related swelling of the retina, a German healthcare cost watchdog said on Monday.

The opinion issued by the German Institute for Quality and Efficiency in Health Care, or IQWiG, could affect the level of reimbursement by public insurers for Eylea in Germany.

IQWiG compared trial data on Eylea and Novartis's Lucentis for treatment of diabetic macular edema (DME) and found there was no relevant difference between the two, neither in terms of how well patients could see nor in terms of side effects.

IQWiG's opinions are taken into account by Germany's medical cost-benefit agency G-BA, which is due to publish an assessment of Eylea's cost-effectiveness in March.

The watchdog has in the past come to similar conclusions on Eylea to treat other eye conditions. In January, it said it was unable to assess whether Eylea was more effective than Lucentis, sold by Roche in the United States and by Novartis elsewhere, to treat macular edema.

And last year, it said it could not assess possible advantages of Eylea to treat wet age-related macular degeneration, a leading cause of blindness in the elderly, because Bayer had not provided relevant data.

Bayer said it would respond to IQWiG's assessment, adding it was convinced of Eylea's benefit to patients.

The drugmaker has said it expects Eylea, also known as VEGF Trap-Eye, to generate more than 1.5 billion euros ($1.9 billion) in peak annual sales, up from 333 million last year.

Bayer HealthCare and Regeneron Pharmaceuticals are collaborating in Eylea's development.

($1 = 0.8048 euros) (Reporting by Frank Siebelt; Writing by Maria Sheahan; Editing by Mark Potter)