The yen has weakened in recent days, edging closer to the psychologically important 160 level against the US dollar. The decline comes despite the widely watched move by Washington to join Tokyo in supporting the currency. Less than two weeks after the US-Japan intervention, the yen has given up roughly half of its gains, raising fresh questions over the effectiveness of coordinated efforts to stabilize the exchange rate.

The yen has lost nearly half of gains from the first joint US-Japan currency intervention since 1998, despite $36.58 billion in spending.

After experiencing a significant drop, the yen managed to stabilize, leaving traders skeptical about the success of the cooperative intervention from the U.S. and Japan. In…