In my experience of dealing with suicidal people at work people who take pain killer overdoses tend to be the more impulsive type of suicidal patient. A significant number of whom, while feeling like they want to die in the moment they take the pills, are likely to change their mind. Perhaps this is partly because death is not instantaneous with painkillers. Regardless, once the tablets are taken its very difficult to reverse the effects, and obviously the larger the quantity, the harder it is. A significant number of painkiller overdoses I deal with are those impulsively taking an overdose after, for example, a drunken arguement with their partner. Most of whom immediately regret it, somecof ehom will finally die of liver or renal problems anyway. Those more 'seriously' suicidal tend to employ less passive methods, more instant methods such as jumping, hanging, and in America, shooting themselves.
You probably won't ever stop someone who is determined to end their life from finally succeeding, but making it harder for people to acquire enough paracetamol to kill themselves gives the impulsive patient time to begin to calm down, and perhaps change their mind, or seek help before they harm themselves. It might also impress on them the seriousness of what they're doing. All of this is, in my opinion, a good thing. Even if it is annoying when I've medium term pain, like a sports injury, and am trying to stock up
