The Strong Potion of Reflection provides 30 pts Reflect Spell for 50 seconds whereas the strongest possible custom potion is 21 pts for 67 secs.

The effects Spell Absorption, Resist Magic and Drain Attribute are not available through custom potions.

Similarly, built-in poisons provide instantaneous damage effects which have a stronger immediate effect than custom poisons, but have a smaller total effect.

The strongest custom Restore Health potion has a magnitude of only 13 pts, but with a duration of 43 seconds (in total, 559 pts of healing). Custom potions can provide a stronger total restore effect, but take longer to do so.

The built in potions provide instantaneous restore effects, for example, the strong Potion of Healing provides an immediate 50 pts of Restore Health.

However, there are some cases where the built-in potions are better: In general, master alchemists with the best equipment can make custom potions and poisons that are much more powerful than the built-in ones: the magnitude and duration are larger, multiple effects can be combined into one potion, and custom potions often weigh just 0.1 pounds. In addition there is a nearly infinite range of custom potions and poisons that the player can make using Alchemy the available custom effects are listed at Alchemy Effects. This is a list of the built-in potions (also beverages and poisons) that are found in stores, on NPCs, and in loot.

Oblivion strong potion of healing