[ That he isn't retching or in pain is a promising enough sign, but at the question, Lambert will simply shrug. It works swiftly for witchers thanks to a more efficient metabolism, but how quickly and how well Allen's body takes to it, well. That will depend on how whatever's in him responds to Swallow.
The potion is more of the sort to restore vitality than to instantly knit flesh. Perhaps the first sign Allen will feel that it's working at all is a warmth that starts spreading from his belly, almost uncomfortable at first, then something that spreads into the rest of his body, gradually making exhaustion fade and replenishing depleted energy. The magic seeks the torn flesh in Allen's body and gives damaged cells a nudge, encouraging new ones to regenerate, mute the pain. It won't patch over his wounds on its own, but who knows how his own biology will interact with the magic. ]
feel free to make up how well it does/doesn't work because GESTURES AT D.GRAYMAN CANON
The potion is more of the sort to restore vitality than to instantly knit flesh. Perhaps the first sign Allen will feel that it's working at all is a warmth that starts spreading from his belly, almost uncomfortable at first, then something that spreads into the rest of his body, gradually making exhaustion fade and replenishing depleted energy. The magic seeks the torn flesh in Allen's body and gives damaged cells a nudge, encouraging new ones to regenerate, mute the pain. It won't patch over his wounds on its own, but who knows how his own biology will interact with the magic. ]