Have started taking agnus castus for pmt and it says to take 2 twice a day- they are 400mg is this too much?

Hard to say: Over the counter herbal products create some challenges. Each company has a different quality of product. Each product has a different amount of the active ingredients. The type of product (capsule, tincture, etc) are quite different. In general the dosing of such products is not as dangerous in terms of overdose as they are often taking for effect. Agnus castus is "relatively" safe, so likely ok.

Vitex agnus-castus: Vitex agnus castus, or chasteberry, can be highly effective in premenstrual symptoms. I agree with dr. Blackwelder that preparations differ, and dosing depends on the company & whether they're tinctures or capsules, etc. Many are standardized to agnusides & aucubin. Use an organic product like gaia herbs -- important since you're taking this daily over at least 3 months, to review effects.