The one-off $131 million impairment charge for Australian Pharmaceutical Industries, which has a market capitalisation of $272.1 million, led the drug wholesaler's share price to fall as much as 13.3 per cent on Thursday.

Shares in API, which were in a trading halt since the beginning of the week, retraced their losses and closed flat at 56.5¢.

API, which also owns the Pharmacist Advice brand, said $52 million of its write-down was for bad debts for its retail customers. A further $20 million was for the underperforming Clifford Hallam Healthcare, while $44 million was a write-down of the value of their retail businesses.

Another $15 million was due to the strong New Zealand dollar's impact on its New Zealand manufacturing unit.

The sector has been hurt by falling revenue from prescription drugs, with changes in the $9 billion Pharmaceutical Benefits Scheme pushing down the cost of generic drugs and reducing sales for retailers and wholesalers.

Drug wholesalers such as API, Sigma Pharmaceuticals and Symbion, which is owned by Ebos, had shifted from a distribution model to a more retail-focused model, Bell Potter Securities senior healthcare analyst John Hester said. ''The industry as a unit has become far more financially disciplined in order to preserve the value of their gross profit from wholesaling in the face of these generic medicines and PBS price reform,'' he said.

''The bad old days of the race to the bottom on price are well and truly behind us and that's been the fundamental change of the industry in response to all these other pressures that they are under.''

Mr Hester said the write-downs had not changed his long-term outlook for API.

API chief executive Stephen Roche said he did not expect further significant impairments. He said the board had ''taken a prudent and conservative approach''.