McNeil Consumer Healthcare, the over-the-counter division at Johnson & Johnson, said the plastic particles in Motrin — which were about the size of a poppy seed — originated at a third-party manufacturer of Motrin’s active ingredient, ibuprofen. The company did not name the manufacturer.

The company said in both cases that the risk to patients was low and it had received no reports of serious harm.

Still, some experts said the two recalls, announced over such a short span, raised questions about how well the company has improved its oversight after a string of manufacturing problems threatened its image as one of the world’s most trusted brands. The company has recalled everything from Tylenol to contact lenses and artificial hips in recent years, and is operating under a consent decree with the Food and Drug Administration in which it has promised to overhaul production at three manufacturing plants. One plant, in Fort Washington, Pa., has been closed since 2010.

“Even the most careful company is occasionally going to have a recall,” said Erik Gordon, who teaches business at the University of Michigan and follows the pharmaceutical industry. But given Johnson & Johnson’s history, he said, the recalls indicate “they’re not there yet. They have not repaired the damage that was done to Johnson and Johnson’s quality control infrastructure.”

Ernie Knewitz, a spokesman for Johnson & Johnson, said the company had been working to improve quality by creating a single, streamlined supply chain and shifting focus to the early detection of potential problems. This heightened attention, he said, has led to several product recalls. He added, “Our goal is to minimize recalls, and yet when we recall a product, we are acting in the best interest of the consumers of our products.”