You influence the actions of the target creature by suggesting a course of activity (limited to a sentence or two). The suggestion must be worded in such a manner as to make the activity sound reasonable. Asking the creature to do some obviously harmful act automatically negates the effect of the spell.
The suggested course of activity can continue for the entire duration. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do. You can instead specify conditions that will trigger a special activity during the duration. If the condition is not met before the spell duration expires, the activity is not performed.
A very reasonable suggestion causes the save to be made with a penalty (such as –1 or –2).
There has been a lot of debate regarding how to regulate the suggestion spell. I have heard arguments that make this spell almost completely useless!
One example is that if you say "its quite hot out here. You should jump into that lake to cool off". This would be deemed an obviously harmful act to a fully armored individual who would thus ignore it. Another example "Lets settle this like men and not cast spells for this combat" would also be deemed an obviously harmful act because a spellcaster would certainly lose in this type of fight. There are many other examples where the implication of the suggestion would probably be harmful to the target individual and people would argue that the suggestion should be ignored.
I disagree with these people. The description of the suggestion spell is quite clear. "The suggestion must be worded in such a manner as to make the activity sound reasonable. Asking the creature to do some obviously harmful act automatically negates the effect of the spell". Therefore, the issue should be wording not the more complicated implications that the suggestion entails. Suggestion is a mind-effecting compulsion spell, not a statement that the target can analyze and decide whether or not he/she should follow.
If I say "you should kill your allies", this should be ignored because killing your allies is obviously a harmful act. But if I say "These people you came here with are savages and are trying to kill us because they are bloodthirsty! You should help us defeat them and we will spare your life", this would be a valid suggestion. At this point, if the target fails his/her save, he/she would follow the suggestion until it is completed or until the duration ends. If while trying to complete the suggestion the target encounters resistance (e.g. while trying to dig a hole to bury his allies the target is attacked) the target must do whatever he can within his power to finish the suggestion (it would be reasonable that this person defend the hole he is trying to dig by fighting off the enemy).
Understandably, the description of this spell in pathfinder makes the interpretation a bit abstract. 3.5 did a much better job with the description.... so here it is
You influence the actions of the target creature by suggesting a course of activity (limited to a sentence or two). The suggestion must be worded in such a manner as to make the activity sound reasonable. Asking the creature to stab itself, throw itself onto a spear, immolate itself, or do some other obviously harmful act automatically negates the effect of the spell. However, a suggestion that a pool of acid is actually pure water and that a quick dip would be refreshing is another matter. Urging a red dragon to stop attacking your party so that the dragon and party could jointly loot a rich treasure elsewhere is a likewise reasonable use of the spell’s power.
The suggested course of activity can continue for the entire duration, such as in the case of the red dragon mentioned above. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do. You can instead specify conditions that will trigger a special activity during the duration. For example, you might suggest that a noble knight give her warhorse to the first beggar she meets. If the condition is not met before the spell duration expires, the activity is not performed.
A very reasonable suggestion causes the save to be made with a penalty (such as –1 or –2) at the discretion of the DM.
This is a much clearer description which shows that suggestion is a compulsion with a consequence dependent on the wording.
If this isn't argument enough, I have listed a few other reasons as to why DMs should not be so stingy in regards to the suggestion spell:
1) it is a 3rd level spell (here is a list of some 3rd level and lower enchantments which are quite bad ass: confusion, cacophonous call, hideous laughter, touch of idiocy)
2) it is language-dependent. Right there, if you do not speak every language there are many creatures who are automatically immune to this spell. Also, many creatures cannot understand language due to low intelligence, thus making them immune
3) Since it is language-dependent, it can be countered by the bardic performance 'countersong'
4) since it is mind-effecting, many creatures are immune to it. It can be undone with a break enchantment and mind blank makes a person immune for 24 hours (i.e. since so many are immune to these spells, they should be more bad ass in general)
4) dominate person is a 5th level spell which allows you to completely own a person for 1 DAY/level (note: dominate person is also a mind effecting compulsion spell... just saying) while suggestion allows you to dominate someone for one activity and lasts less long (3rd level vs 5th level difference in power. Seems legit)
For these reasons I feel like DMs shouldn't be so stringent on this spell. I would like to get feedback from people to hear your opinions, but please read my full argument and give it some thought before you start to criticize.