* FTSEurofirst 300 down 0.3 percent
* Pharmaceuticals weak, led by AstraZeneca
* Euro STOXX 50 technical analysis bullish
By Tricia Wright
LONDON, Dec 13 (Reuters) - European shares dipped on Thursday as new stimulus steps from the Federal Reserve were overshadowed by concern over U.S. austerity measures, though traders made light of the falls after a recent powerful rally.
The FTSEurofirst 300 was 0.3 percent lower at 1,136.29 by 1159 GMT, slipping after a steep three-week rally which had seen the index rise in 16 out of 18 sessions propelling it to 18-month highs.
Trading volumes were extremely thin, at a third of the 90-day daily average, exaggerating any moves on the index.
The Fed announced a new round of monetary stimulus on Wednesday and indicated interest rates would remain near zero until unemployment falls to 6.5 percent.
But a key issue continued to be worries over the U.S. budget impasse, and whether the United States would miss a year-end deadline to avert the "fiscal cliff" of some $600 billion of tax hikes and spending cuts due to start in January.
Fed Chairman Ben Bernanke warned on Wednesday that monetary policy would not be enough to offset damage from the fiscal cliff.
"There's definitely some profit-taking in play and the optimism that the fiscal cliff can be averted is drying up now, but with markets sitting at or around recent highs, any uncertainty is going to see traders reining it in," said Mike McCudden, head of derivatives at Interactive Investor.
Andy Ash, head of sales at Monument Securities, said: "I think most people are trying to keep everything pretty tight and pretty flat for the rest of the year, and at the moment we don't see any significant reason why they shouldn't be able to do that."
"It's not really a fiscal cliff; it's more of a fiscal slope. Even if they miss it by a month or two it's not going to have the inherent damage instantaneously that people expect."
Weak drugmakers were the main drag on the index, led by a 2.3 percent drop from AstraZeneca in brisk volume after it said an experimental rheumatoid arthritis drug proved inferior to Abbott Laboratories' Humira in a clinical study.
The news dampened hopes for one of the few late-stage products in the company's pipeline.
Trading volume in AstraZeneca was 113 percent of the 90-day daily average.
The euro zone's blue-chip Euro STOXX 50 was down 0.1 percent at 2,627.90, having hit a new closing high for the year on Wednesday, at 2,630.34.
Even with the index close to "overbought" territory - its relative strength index, a closely watched momentum indicator, is just below 70 - some technical analysts remained optimistic.
"As long as the former mid-term top around 2,610 (former 2012 high) is support, we remain bullish," said Philippe Delabarre, a technical analyst at Trading Central.
Delabarre targets 2,638, the day's high, and 2,650, a psychological resistance level.