Shares in Salix Pharmaceuticals surged more than 18 percent Wednesday after the company disclosed a drug licensing deal, a partnership agreement and positive test results.

Salix (Nasdaq: SLXP), which focuses on gastrointestinal drugs, shares climbed more than $2 a share to $13.39 in trading through late Wednesday afternoon. All the news announcements were made before the markets opened.

However, by the end of the day Salix was up only 21 cents to close at $12.96.

In after-hours trading, Salix shares plummeted 13 percent, or $1.64, to $11.32.

On Thursday, the stock rebounded to trade  at $13.36 in afternoon trading.

Despite the good news on Wednesday, analysts at Pacific Growth Equities downgrade Salix to "neutral" from "buy" on Thursday.

Wilmington Pharmaceuticals has licensed to Salix exclusive rights to a treatment for diabetic gastric stasis, which is often associated with diabetes. Salix paid an upfront free, which it described only as “small,” for rights to Metoclopramide-Zydis when it receives Food and Drug Administration approval.

The drug is designed to be taken orally and to disintegrate rapidly on the tongue without water. Its formulation is protected by patent through 2022.

“Salix is expanding its product portfolio with the acquisition of a fast-dissolving formulation of metoclopramide,” said Carolyn Logan, chief executive officer at Salix. “Market research indicates a high level of demand for a patient-friendly formulation of this widely-prescribed agent. We believe the availability of metoclopromide-Zydis should serve to increase patient compliance and, in many instances, provide an economical alternative to emergency room visits and/or intravenous infusion by patients suffering from nausea and gastric distress.”

Salix also disclosed a co-promotion agreement with Eisai, which will utilize some 600 sales representatives to promote the Salix product Colazal. Financial terms were not disclosed.

Colazal is used to treat ulcerative colitis.

In clinical trials, Salix reported positive results from the first of two Phase III clinical trials for a treatment called mesalamine. It is intended to maintain remission in patients who have ulcerative colitis.

Salix also described as “successful” a PhaseIIb trial for rifaximin, which is intended to treat irritable bowel syndrome. Salix licensed rights to the drug from Italian pharmaceutical firm Alfa Wassermann S.p.A.