I have a 5-yr-old and 3-yr-old, both of whom have spat out medicine since age 1, and no amount of bad cop (pinning down, nose pinching, etc) or good cop (bribes, sweeties, chocolates) behaviour helped. Since then, I've become an expert medicine hider! It can be done. Believe me, if my boys think there's a hint of medicine in their juice or food, it gets spat out.

My tips:
for fevers/pain: use panadol suppositories, available over the counter from the pharmacist in Watsons/Mannings. The brand is Ben-u-ron. There are two strengths depending on age of child. Again, they are (understandably) very resistant to this, so I do it while they are asleep or if wiping their bottoms after they've been to the toilet. A latex glove, a bit of vaseline, it's over in a second and they might squawk, but it's too late then!! ;-)

for colds: Polaramine (again, over the counter from pharmacist). It's an antihistamine, so dries up the runny nose - has very little taste and you only need a tiny amount (1.25ml for age 2-6). Very easy to hide in a Petit Miam yoghurt (mix thoroughly) or in a strong-tasting juice like Ribena

Calpol: blackcurrant flavour, hide in Ribena
Nurofen: orange flavour, hide in orange juice or pour over a chopped up orange

Some other tablets can be crushed up and hidden in ketchup or baked beans.