Why you need to approve tokens before swapping on a DEX

05 September 2026

You click "Swap" on a DEX like PancakeSwap. Nothing happens. Your wallet asks you to approve something first. This is not a bug or an extra step developers added to annoy you. It is a deliberate security feature built into the ERC20 standard.

How token approval works

Your tokens live on the blockchain inside your wallet address. A DEX contract cannot simply reach in and take them. That is the entire point of a self-custodial wallet. No one moves your assets without your permission. But a swap also cannot work if the DEX contract has no access to your tokens at all. The contract needs to pull your tokens and push the swapped tokens back.

The approval transaction gives the DEX contract permission to spend a specific amount of a specific token from your wallet. Only you can sign this. Only the contract you specify can use it.

One approval per token

You only pay the gas cost for this permission once per token. After you approve MILK on PancakeSwap, you do not need to approve it again when you swap MILK a second time. If you want to swap a different token on the same DEX, you approve that one separately. Each token requires its own approval transaction.

This is worth remembering. If you approve a token once and then swap it repeatedly over weeks or months, you avoid paying that gas fee again. The initial cost is a one-time setup.

Limited vs. unlimited approval

You can approve two ways.

The safer way is to approve the exact amount of tokens you plan to swap. The contract can then spend only that number. If you want to swap 1000 MILK, you approve 1000 MILK. That is called a limited approval.

The faster way is to approve an extremely large number - often the maximum your wallet allows. This is an unlimited approval. It means the contract can spend any amount of that token from your wallet, any time, until you revoke permission. It saves you from having to approve again if your swap amount changes. But it also means you are trusting that contract completely.

The security risk

An unlimited approval is only as safe as the contract you approved. If that contract has a vulnerability, or if it is a malicious contract disguised as a DEX, someone can drain every token of that type from your wallet. You gave them the key. They can use it.

This is not theoretical. Unlimited approvals have been exploited repeatedly. A compromised frontend or a malicious upgrade to the contract can turn a trusted approval into a theft vector. Limited approvals reduce this risk because the damage is capped at the approved amount.

Revoke.cash as a management tool

You can check your approvals at any time using tools like Revoke.cash. It scans your wallet for active approvals on multiple chains and lets you revoke them - removing the permission the contract had. This costs gas but closes the open door.

If you have old unlimited approvals you no longer use, revoking them is good hygiene. It costs nothing to leave them open until they get exploited. It costs gas to close them now. Choose which risk you prefer.

Two transactions, not one

Approval and swap are separate on-chain transactions. Each goes into a block and costs its own gas.

The approval transaction says: "I give contract X permission to spend Y amount of token Z." The swap transaction then says: "Use that permission. Swap Y of token Z for the other token."

If you do not approve first, the swap transaction will fail. The contract cannot move your tokens. Many wallets now bundle approval and swap into a single user action. Behind the scenes, both transactions still execute. The wallet just handles the sequence for you.

Why this design exists

The approval mechanism exists because blockchain wallets are not bank accounts. Banks can reverse transfers. They can freeze assets. None of that is possible on a decentralized exchange. The approval gives you explicit, granular, revocable control before any contract touches your funds.

It is one extra click. It costs gas. But it also means no contract can drain your wallet without your signature. That is the trade-off built into self-custody. Approval is not a flaw in the design. It is the design.

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