Risk and benefit. Every medicine has it. And it's been brought into sharp focus by the rumble over AstraZeneca's COVID vaccine.
Several countries have reported people developing blood clots soon after having the jab and have suspended the rollout as a precaution while medical regulators investigate.
According to the European Medicine's Agency, there have been 30 cases in five million vaccinated people across the continent.
But blood clots are common - around 1 in 1,000 people a year develop one because of medical conditions or medication, including the pill.
What about certain types of blood clots?
The German and Norwegian health authorities took action because of reported clots causing brain haemorrhages in relatively young people in the days after vaccination.
It's called cerebral venous sinus thrombosis (CVST) - and in Germany there have been seven cases of out of 1.6 million vaccinations.
That's a rate of 4.5 per million people.
And that does look a little higher than you would expect. Doctors estimate the background rate of CVST in people under 50 would be one or two a month per million people.
But those numbers are small and they still don't prove a causal link with the vaccine.
Against any possible risk you have to weigh up the benefits.
COVID kills. Even in middle-age men the mortality rate is about 1,000 deaths per million infections. Death the vaccine largely prevents.
Still torn by the risk and benefit?
Bear this in mind.
Paracetamol, a drug so many of use regularly. Well it can cause liver failure if the maximum daily dose is regularly exceeded by just a couple of tablets.
A very real safety risk.
As for AstraZeneca's jab, the European medicines regulator is looking at the individual reports of clots to see if there really is good reason to stop the rollout of a life-saving vaccine.