Is the homebrew weapon attack cantrip 'Viper Strike' balanced?

Question

I have created some new cantrips which include making a weapon attack as there are only two official ones to choose from. I've tried to follow these principles based on Booming Blade and Green-Flame Blade:

• Melee weapon attack only, 5 foot range
• No unavoidable extra damage to the primary target at level 1
• Damage no more than 2 targets
• Spellcasting ability modifier doesn't have to affect the spell at all
• Only available on the Warlock, Sorcerer and Wizard spell lists

But with such a small amount of official content to compare them against, it's hard to say if they're balanced.

Is this spell balanced with Booming Blade and Green-Flame Blade?

Viper Strike

Conjuration cantrip

Casting Time: 1 action
Range: 5 feet
Components: V, M (a weapon)
Duration: Instantaneous

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.

On a hit, the target suffers the attack's normal effects, and it must make a Constitution saving throw or take 1d8 poison damage.

At 5th level, the melee attack deals an extra 1d8 poison damage to the target, and the damage the target takes if it fails its saving throw increases to 2d8. Both damage rolls increase by 1d8 at 11th level and 17th level.

Answer 1

Slightly OP, but pretty close.

Both BB and GFB have 'triggers' that must be passed in order to activate the extra damage (moving and another target in range). This has a similar damage addition with a similar trigger requirement (making a save.)

However, with BB, they simply choose not to move. They can't just not to make the save here. Because of that, I'd recommend dropping the damage down to D4s(scaling).

Answer 2

I think BB is slightly weaker than this cantrip overall, because you "choose" to suffer or not the extra damage. You can balance this by making the cantrip consume the bonus action as well or by lowering the scaling.

But if a significant part of the enemies in your campaign will have resistance to poison (like undead), you can leave it as it is.