Even though you may have taken a price, the odds at which your bet is settled may be adjusted if certain conditions apply.
Non-runners
A non-runner is a horse that does not take part in the race. A horse may be withdrawn any time up to the off of a race. If you bet on a non-runner on the day of the race, your stake will be refunded. If, however, you bet ante-post, you will lose your stake.
If a horse comes under starter’s orders in a race but refuses to run, it is not considered a non-runner. It is important to check the results because a horse can be withdrawn right up to the last minute. You may see it set off for the start, but it may, for example, be withdrawn because it refuses to go into the starting stalls.

Withdrawn horses
Rule 4 deductions
After a book has been made on a race, a horse may be withdrawn. This will mean that the prices offered on the race will be incorrect, as they would have been worked out including the withdrawn horse. The bookmakers are therefore allowed to make an adjustment to the prices, called a rule 4 deduction (usually abbreviated to R4). The deduction is quoted as an amount in the pound, for example 10p in the £. The amount of the deduction is determined by the price of the withdrawn horse (see below table). The deduction applies to both winning and placed horses. Only the winnings are affected and not your stake.

Odds of horse withdrawn                                        Deduction            % of winnings

<= 3/10             75p in £                                          75
Over 3/10         up to and including 2/5                70p in £              70
Over 2/5           up to and including 8/15              65p in £              65
Over 8/15         up to and including 8/13              60p in £              60
Over 8/13         up to and including 4/5                55p in £              55
Over 4/5           up to and including 20/21            50p in £              50
Over 20/21      up to and including 6/5                 45p in £              45
Over 6/5     up to and including 6/4                       40p in £             40
Over 6/4     up to and including 7/4                       35p in £              35
Over 7/4     up to and including 9/4                       30p in £              30
Over 9/4     up to and including 3/1                       25p in £              25
Over 3/1     up to and including 4/1                       20p in £              20
Over 4/1     up to and including 11/2                     15p in £              15
Over 11/2     up to and including 9/1                     10p in £              10
Over 9/1     up to and including 14/1                     5p in £                 5
> 14/1                                                                        no deduction

Example
A horse that was 4/1 in the betting is withdrawn. The SP of the winning horse is 2/1. The price will be subject to a rule 4 deduction of 20p in the £.
A £10 bet tax paid on a winning horse at 2/1 would normally return £30. With a rule 4 deduction of 20p in the £, the bet would return £26.
Calculation: (20 - 20%) + 10 = 26

If there is sufficient time before the race is off, a new book will be made. In this case, the rule 4 deduction will apply to prices taken before the time that the new book was made. Prices taken on the new book and SP prices will not be subject to a rule 4 deduction.
Each way bets
Horses being withdrawn can also affect the odds paid and the number of places paid for each-way bets. When you placed your bet there may have been eight runners, meaning you would expect to be paid if your horse came third. However if a horse is withdrawn, the third-placed horse no longer counts, as there will only be seven runners. You will only get paid if your horse is first or second.
Dead heats
Occasionally two horses or more are declared the winner. This is termed a dead heat. When a horse dead heats for first place, the returns are halved. If there are three horses in the dead heat then the returns are divided by three. If placed horses dead heat’ the returns are only reduced if there are more places than usual being paid. For example, if two horses dead heat for second place and three places would normally be paid, the returns will not be affected. If, however, only two places would normally be paid and two horses dead heat for second place, the returns of the placed horses will be affected. So too will the returns of any each-way bets on the winner.