what effects should i expect? Any negative ones?
Codeine can definitely have unwanted side effects. Some people are also allergic or sensitive to codeine, and it can be tricky because the allergy doesn't normally show up until you've done it a few times. Some people are poor metabolizers of codeine and find that they get few pleasant effects from it but may still get the side effects. Effects that are normally considered desirable include pain relief, euphoria, decreased anxiety and sedation/drowsiness (although that one can also be unwanted).
Common side effects include itching, nausea/vomiting, dry mouth, constipation, difficulty urinating, and feeling apathetic or depressed.
Symptoms of a codeine sensitivity include nausea, headaches, heart palpitations, disorientation, irritability/agitation, dizziness, flushing, and visual disturbances.
Symptoms of a codeine allergy include swelling, hives or rash, sweating, difficulty breathing, heart palpitations, convulsions and heart troubles.
Of course some of the symptoms of a sensitivity or allergy can also be normal side effects and vice versa. If you end up finding you get a lot of side effects or little pleasurable effects then maybe it isn't for you.