The first answer was correct:
IR = Instant release. Drug will take effect from 5-30 minutes after ingestion and it will stay effective for 2-6 hours.
ER/SR = Extended or Sustained Release. Drug will take effect from 15-60 minutes after ingestion and it will stay effective for anywhere from 4-12 hours from ingestion.
Breaking/cutting/crushing an IR tablet can make it take effect faster, but possibly shorten (slightly) how long it is effective.
One should NEVER break/cut/crush an SR tablet as they will get a MUCH higher dose than is on the label. It WILL take effect faster, however it will NOT last nearly as long.
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