The delay was announced as two major equestrian nations formally stated their position on the progressive list - the British Equestrian Federation against it and the United States Equestrian Federation in favour.
The progressive list, narrowly passed by the FEI's General Assembly last month, allows for small amounts of several common therapeutic drugs in a horse's system, including the powerful anti-inflammatory phenylbutazone.
The organisation acknowledged that there was clearly a need for further debate on the issue.
The FEI said early today (NZ time) that its bureau had agreed to delay implementation of the new drugs policy until April 5 next year.
It said the delay was to allow broader debate and consultation on the policy change approved by the General Assembly, in particular over the restricted use of a small number of non-steroidal anti-inflammatory drugs (NSAIDs).
"The FEI recognises that a number of issues arise as a result of this policy change, which was voted in by a majority of 53-48 at the meeting in Copenhagen on November 19," it said in a statement.
"The change in policy will affect national federations and organisers, who must comply with state laws as well as international competition rules.
"It has implications for the breeding industry and the selection of horses and may impact on all those concerned with animal welfare.
"It also affects the laboratories involved in the testing procedures."
The FEI ackowledged that the decision, which reversed the 1993 ban on phenylbutazone, had provoked "considerable debate among FEI stakeholders".
"The FEI acknowledges that these concerns are all legitimate and feels that there is clearly a need for further debate on the issue," it said.
"The delayed implementation of the new Equine Prohibited Substances List will also allow for supplementary research to be carried out on the use of NSAIDs in the competition horse."
FEI president Princess Haya said: "The FEI has been criticised for not providing sufficient time for consultation on the substances that differentiate the new policy from the old and there has also been widespread unease about the late publication of the progressive list.
"Views that have been expressed since the vote are being taken extremely seriously by the FEI as legitimate welfare concerns and we give similar importance to our membership's decision to effect a policy change," she said.
"In light of both these considerations, we felt it was only fair to delay implementation of the new list to allow everyone to have their say and let other veterinary experts look at the science behind this policy change.
"The controversy surrounding the [progressive] list has almost completely overshadowed the clean sport campaign, which received overwhelming support at the General Assembly.
"Now we can allow the focus to return to the wonderful work that has been undertaken by the joint commissions chaired by Professor Arne Ljungqvist and Lord Stevens, which has provided a clear roadmap for the delivery of clean sport."
The FEI said the statement from the British and United States equestrian federations reflected the ongoing debate on the issue.