Twitter Inc.’s (TWTR) stock has surged about 82% in the past six months, but Macquarie Research believes the stock's upside is limited.

The firm lowered its rating on Twitter shares to Neutral from Buy on concerns that the current valuation is too high. Its price target was raised to $42 per share from $36. Twitter was down more than 1% near $44.25 in pre-market trade Wednesday.

"While product improvements are a positive for current users, we don't see it having dramatic impacts on Twitter's ability to attract new users," Macquarie Research analysts said in a note.

In April, Macquarie Research raised its rating to Buy on Twitter, citing buying opportunities as the company drew negative headlines that weighed on shares. Since then, shares have surged 50%.

At the time, Macquarie noted that Twitter was improving its profitability and had strong momentum.

"While we expect business fundamentals to continue to improve ... the valuation, after the recent rise in the stock, will likely limit upside from current levels," Macquarie said.

Other Street Sentiment on Twitter

In other recent analyst moves on Twitter, Goldman Sachs said last week that it sees a 25% upside in the stock. It reiterated a Buy rating and increased its price target to $55 from $40, saying Twitter is successfully monetizing engagement. (See also: Twitter Has a 25% Upside: Goldman Sachs.)

“Twitter continues to build on ‘Information Quality’ efforts they first spoke about on the fourth-quarter earnings call by moderating unwanted behavior, spam accounts, and low quality tweets through product innovation, acquisition, or more active removal of violating accounts and developer applications,” said Goldman Sachs analyst Heath Terry in a note