This year's Japanese stock rally has a surprise leader: pharmaceutical companies, which tend to lag behind the market when it gains.

Drugmakers soared 22 percent, the most among the 33 industry groups on the benchmark Topix index, compared with a 10 percent gain for the Topix itself.

The Topix Pharmaceutical Index climbed to its highest on record on March 24, and earnings multiples are about twice the market average. The Topix has beaten pharmaceutical shares in 12 of the past 17 years it rose.