[AbiDrew85]

I would be careful of putting bulk spearmint leaves in capsules, if they contain any seeds they can be poisonous.

in a tea they won't be.

Dried, crushed, and sorted, made to be used as food in any of many ways including as a raw garnish, I highly doubt that should be a problem. The link I provided was such. From Angelina's Gourmet. Having ordered a bag of it myself and done some repacking, I can verify there have so far been no seeds. Not as I've been measuring out doses either.