SAN DIEGO ( TheStreet) -- Jefferies analyst Thomas Wei is still an Arena Pharmaceuticals ( ARNA - Get Report) bull, just not as much after slashing his sales forecast for the yet-to-be-launched obesity pill Belviq. Here's what Wei writes Wednesday:

We remain confident in the long-term potential for Belviq based on combination use with phentermine, but we are lowering our PT from $20 to $14, as we moderate our launch expectations for Belviq monotherapy following disappointing sales for competitor Qsymia.

"Moderate." That's sell-side analyst speak for, "Oh my god, my old forecast is so wrong."

Wei's "moderated" guess for Belviq sales in 2013 is now $35 million, down from $115 million. That's 70% "moderation" to his estimate, by the way.

In 2015, Wei now projects Belviq sales of $365 million, down from $700 million.

Vivus' ( VVUS - Get Report) poor third-quarter financial report and honest (but generally downbeat) assessment of the challenges facing peddlers of new obesity pills has analysts and some investors in re-assessment mode.

Credit Suisse analyst Lee Kalowski was actually ahead of his peers warning about Vivus' slow Qsymia launch but he, too, had more cutting to do Wednesday. His new Qsymia 2013 sales forecast is $88 million, down from $165 million. He's not much more optimistic about Arena and Belviq:

We have updated our model, though we leave Belviq revenue forecasts unchanged, resulting in higher launch forecasts for Belviq relative to Qsymia. Belviq will have no REMS and will be available at the retail channel, but we are concerned that Eisai is launching Belviq initially with (a) an equally under-sized field force and (b) weaker monotherapy efficacy that leaves it positioned somewhat awkwardly in the market. Combination trial data is unlikely until at least 2015, which will prove too late if the initial launch underwhelms. Belviq's monotherapy response rate is around 40%, which could hinder the uptake of repeat use -- if the product is used in a similar manner as Qsymia, with many physicians putting only a few patients on the product at a time. Despite having split economics on Belviq, ARNA now trades at a substantial premium to VVUS.

The most worrisome disclosure by Vivus Tuesday was the fact that 30% of patients willing to try Qsymia and who get actually get a prescription from their doctor are not following through because the drug costs too much out of pocket. Insurance coverage for the drug is only 20% currently, not the 50%-plus estimated by third-party research firms. This trend is not a friend for Belviq, which is much less effective.

J.P. Morgan analyst Cory Kasimov writes Wednesday:

The Arena conference call was relatively uneventful in terms of providing additional details on the launch plans for Belviq (obesity). However, we see few signs that the early launch of Belviq will avoid the problems VVUS is currently experiencing with its weight loss drug, Qsymia (including slow uptake and a meaningful number of ptatients not filling their prescription due to a relatively high out-of-pocket cost). In fact, Belviq could well experience a more lackluster launch vs. Qsymia given the drug's more modest efficacy. These potential issues combined with minority economics currently prevent us from being more constructive on the name at this valuation.

Kasimov forecasts 2013 Belviq U.S. sales of $74 million growing to $481 million in 2016 - in other words, no blockbuster, particularly since Arena only gets approximately one-third of the drug's economics, the rest going to Eisai.

If you haven't already, read TheStreet contributor Nate Sadeghi's take on Arena and Vivus. The headline says it all, Arena Pharma Is Still A Good Short.

-- Reported by Adam Feuerstein in Boston.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.