$40-50 MILLION IN CUBICIN SALES PROJECTED FOR 2004.

Cubist Pharmaceuticals, Inc. (Nasdaq: CBST) has provided updated information on the launch of its flagship product, Cubicin (daptomycin for injection). CUBICIN was launched in November 2003 in the U.S. after receiving FDA approval for the treatment of complicated skin and skin structure infections caused by certain Gram-positive organisms.

Michael W. Bonney, president & CEO of Cubist, commented: "Based on twenty weeks of sales data since launch as well as recently completed market research, we now feel comfortable providing an expected range for CUBICIN sales this year. Based on our confidence in these data, we believe CUBICIN can reasonably achieve sales in the range of $40-50 million for 2004."

In addition to its revenue guidance for the year, Cubist provided additional data from Cubist-sponsored market research conducted over the first two months of this year. These data is as follows:

There is high awareness of Cubist amongst its targeted physician and pharmacist base, with respondents mentioning Cubist second only to Pfizer Inc. as a respected developer of agents to treat Gram-positive infections and ranks Cubist highest among emerging companies in the antiinfective space. There is also high awareness of CUBICIN amongst the targeted physician and pharmacist base, with infectious disease physicians indicating 95% awareness and hospital pharmacists indicating 84% awareness.

The main product messages delivered by Cubist's 75 sales representatives appear to be well received and well understood by physicians; cited most frequently were CUBICIN's efficacy against methicillin-resistant Staphylococcus aureus (MRSA), its bactericidal activity, its convenient once-daily dosing regimen and its safety profile.

Cubist also reported the following:

Almost 1,200 hospitals and institutions have now ordered CUBICIN.

Of the roughly 900 institutions that Cubist targets, about 60% have now ordered CUBICIN.

Re-order rates continue to rise, with 65% of institutions re- ordering to date; since late January, the number of new ordering institutions is averaging approximately 75 accounts per week.

Re-orders of CUBICIN continue to be roughly 50% larger than initial orders.

Of the roughly 900 accounts that Cubist targets, reports indicate that only about 35% have met and/or made a decision on whether to add CUBICIN to formulary.

According to company sales representatives, 96% of the committees that have met and made a decision on CUBICIN have placed the drug on formulary; notably, the majority of the 4% of the hospitals that have declined to place CUBICIN on formulary have purchased the drug nonetheless. Sales representatives also indicate that 21% of all formulary acceptances have not restricted the use of the drug; of those that have, the majority have restricted use to infectious disease (ID) doctors, consult by an ID doctor, or use by critical care physicians, as expected with a new, first-in-class IV antibiotic, and consistent with the types of physicians prescribing other hospital-based Gram-positive antibiotics, including vancomycin.

Outpatient facility use continues to exceed Cubist's expectations and represents about one third of CUBICIN orders; Cubist market research indicates that CUBICIN's once-daily dosing gives it a distinct benefit over other treatments in this setting.

Cubist also reiterated the following information:

Enrollment in the ongoing Phase 3 study of CUBICIN in the treatment of endocarditis/bacteremia is continuing, and the company expects to report data from this study in the first half of 2005.

Cubist plans to conduct additional studies on CUBICIN during 2004 in renally impaired patients and in patients suffering from febrile neutropenia, osteomyelitis, surgical wound infections and other indications.

Cubist recently launched CORESM (CUBICIN Outcomes Registry and Experience), a multicenter retrospective registry program designed to collect information on patients who have received CUBICIN therapy; data from the registry will be made available to participants and published periodically.

Cubist and its partner Chiron Corporation expect to announce the European regulatory strategy for CUBICIN in the first half of 2004.

Cubist is focused on three key issues in 2004 and beyond: successfully commercializing CUBICIN in the U.S. and supporting its partners worldwide, expanding and progressing the company's pipeline and improving the company's capital structure.

On Cubicin (daptomycin for injection)

To reduce the development of drug-resistant bacteria and maintain the effectiveness of CUBICIN, CUBICIN should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria susceptible to CUBICIN.

For full prescribing information, visit http://www.Cubicin.com.

About Cubist

Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development and commercialization of drugs that meet significant unmet medical needs. Since its founding in 1992, Cubist has been exclusively focused on exploiting business and product opportunities in the antiinfective marketplace. In the U.S., Cubist markets Cubicin (daptomycin for injection), the first antibiotic in a new class of antiinfectives called lipopeptides, for the indication of complicated skin and skin structure infections caused by Gram-positive bacteria. Cubicin is currently the only once-daily bactericidal antibiotic approved in the U.S. with activity against both methicillin-susceptible and methicillin-resistant Staphylococcus aureus (MSSA and MRSA). Cubist's pre-clinical pipeline includes an oral version of ceftriaxone (OCTX), a broad-spectrum cephalosporin antibiotic, and other novel members of the lipopeptide class. Cubist is headquartered in Lexington, Mass.

Additional information can be found at Cubist's web site at http://www.cubist.com or call 212/845-4266.