I would probably say either alcohol or microplastics. Both are carcinogenic. At least alcohol is avoidable, but microplastics pretty much saturate all of our environments. It reminds me of when they were doing experiments to figure out the impact of lead, they couldn't even open the door to the lab, because the airborne concentration of lead would throw off the readings. We might not ever know what real health is like without walking around with grams of microplastics inside us.
The biotech making your new drugs follows a less than scientific method. Lots of cherry picking of data, fudging results, etc. Part of me thinks this is part of why a lot of drugs never make it past trials. There is more incentive for individuals to come up with a drug that almost passes trials than to come up empty handed for years.